1. A boat running downstream covers a distance of 16 km in 2 hours while for covering the same distance upstream, it takes 4 hours. What is the speed of the boat in still water?
a) 4 kmph
b) 6 kmph
c) 8 kmph
d) None of these
Discussion
Explanation: Rate downstream
= $$\frac{{16}}{2}$$ kmph = 8 kmph
Rate upstream
= $$\frac{{16}}{4}$$ kmph = 4 kmph
Speed in still water
= $$\frac{1}{2}$$(8 + 4) kmph = 6 kmph
2.The speed of a boat in still water in 15 km/hr and the rate of current is 3 km/hr. The distance travelled downstream in 12 minutes is:
a) 1.2 km
b) 1.8 km
c) 2.4 km
d) 3.6 km
Discussion
Explanation: Speed downstream
= (15 + 3) kmph
= 18 kmph
Distance travelled
= $$\left( {18 \times \frac{{12}}{{60}}} \right)$$ km
= 3.6 km
3. A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:
a) 2 mph
b) 2.5 mph
c) 3 mph
d) 4 mph
Discussion
Explanation: Let the speed of the stream x mph.
Speed downstream = (10 + x) mph
Speed upstream = (10 - x) mph
$$\eqalign{ & \frac{{36}}{{ {10 - x} }} - \frac{{36}}{{ {10 + x} }} = \frac{{90}}{{60}} \cr & 72x \times 60 = 90\left( {100 - {x^2}} \right) \cr & {x^2} + 48x - 100 = 0 \cr & \left( {x + 50} \right)\left( {x - 2} \right) = 0 \cr & x = 2\,\text{mph} \cr} $$`
4. A man can row at 5 kmph in still water. If the velocity of current is 1 kmph and it takes him 1 hour to row to a place and come back, how far is the place?
a) 2.4 km
b) 2.5 km
c) 3 km
d) 3.6 km
Discussion
Explanation: Speed downstream = (5 + 1) kmph = 6 kmph
Speed upstream = (5 - 1) kmph = 4 kmph
Let the required distance be x km
$$\frac{x}{6} + \frac{x}{4}$$ = 1
2x + 3x = 12
5x = 12
x = 2.4 km
5. A boat covers a certain distance downstream in 1 hour, while it comes back in $$1\frac{1}{2}$$ hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?
a) 12 kmph
b) 13 kmph
c) 14 kmph
d) 15 kmph
Discussion
Explanation: Let the speed of the boat in still water be x kmph
Speed downstream = (x + 3) kmph
Speed upstream = (x - 3) kmph
$$\eqalign{ & \left( {x + 3} \right) \times 1 = \left( {x - 3} \right) \times \frac{3}{2} \cr & \Rightarrow 2x + 6 = 3x - 9 \cr & x = 15\,\text{kmph} \cr} $$.
6. The speed of a boat along the stream is 12 km/hr and against the stream is 8 km/hr. The time taken by the boat to sail 24 km in still water is?
a) 2 hrs
b) 4 hrs
c) 2.4 hr
d) 1.2 hrs
Discussion
Explanation: Speed of downstream
D = 12 km/h
Speed of upstream
U = 8 km/h
Speed of boat in still water
$$\eqalign{ & = \frac{{D + U}}{2} \cr & = \frac{{20}}{2} \cr & = 10\,km/h \cr} $$
Time taken by the boat in still water
$$\eqalign{ & = \frac{{24\,km}}{{10\,km/hr}} \cr & = 2.4\,{\text{hours}} \cr} $$
7. A motorboat in still water travels at a speed of 36 km/hr. It goes 56 km upstream in 1 hour 45 monutes. The time taken by it to cover the same distance down the stream will be-
a) 1 hour 24 minutes
b) 2 hour 21 minutes
c) 2 hour 25 minutes
d) 3 hour
Discussion
Explanation:
$$\eqalign{ & {\text{Speed upstream}} \cr & {\text{ = }}\left( {\frac{{56}}{{1\frac{3}{4}}}} \right)km/hr \cr & = \left( {56 \times \frac{4}{7}} \right)km/hr \cr & = 32km/hr \cr & {\text{let speed downstream be }}x{\text{ km/hr}}{\text{.}} \cr & {\text{Then speed of boat in still water }} \cr & {\text{ = }}\frac{1}{2}\left( {x + 32} \right)km/hr \cr & {\text{ }}\frac{1}{2}\left( {x + 32} \right) = 36\,\,\, \Rightarrow x = 40 \cr & {\text{Hence , required time}} \cr & {\text{ = }}\left( {\frac{{50}}{{40}}} \right)hrs \cr & = 1\frac{2}{5}hrs \cr & = 1\,{\text{hour}}\,24\operatorname{minutes} \cr} $$
8.P, Q and R are three towns on a river which flows uniformly. Q is equidistant from P and R. I row from P to Q and back in 10 hours and I can row from P to R in 4 hours. Compare the speed of my boat in still water with that of the river.
a) 4 : 3
b) 5 : 3
c) 6 : 5
d) 7 : 3
Discussion
Explanation:
$$\eqalign{ & {\text{Let PQ = QR = }}x{\text{ }}km \cr & {\text{let speed downstream }} \cr & {\text{ = }}a{\text{ }}km/hr \cr & \,\,\,\,\,\,\,\,\,\, \to \,\,{\text{downstream}} \to \cr & {\text{P}}\overline {\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Q}}\,\,\,\,\,\,\,\,\,\,\,\,\,y\,\,\,\,\,\,\,\,\,\,\,} \,{\text{R}}\,\,\,\, \cr & {\text{and speed upstream }} \cr & {\text{ = }}b{\text{ }}km/hr{\text{ }} \cr & {\text{then, }}\frac{x}{a} + \frac{x}{b} = 10 \cr & \Rightarrow x = \frac{{10ab}}{{a + b}} \cr & {\text{and }}\frac{{2x}}{a} = 4 \cr & \Rightarrow x = \frac{{4a}}{2} = 2a \cr & {\text{from (i) and (ii) we have:}} \cr & 2a = \frac{{10ab}}{{a + b}} \cr & 5b = a + b \cr & a = 4b \cr & {\text{Required ratio }} \cr & {\text{ = }}\frac{{{\text{Speed in still water}}}}{{{\text{Speed of river}}}} \cr & = \frac{{\frac{1}{2}\left( {a + b} \right)}}{{\frac{1}{2}\left( {a - b} \right)}} \cr & = \frac{{\left( {a + b} \right)}}{{\left( {a - b} \right)}} \cr & = \frac{{4b + b}}{{4b - b}} \cr & = \frac{5}{3} \cr} $$
9. A boat moves downstream at the rate of 1 km in $${\text{7}}\frac{1}{2}$$ minutes and upstream at the rate of 5 km an hour. What is the speed of the boat in the still water?
a) 8 km/hour
b) $${\text{6}}\frac{1}{2}$$ km/hour
c) 4 km/hour
d) $${\text{3}}\frac{1}{2}$$ km/hour
Discussion
Explanation: Rate downstream of boat
$$\eqalign{ & {\text{ = }}\left( {\frac{1}{{\frac{{15}}{{2 \times 60}}}}} \right)\,{\text{kmph}} \cr & = \frac{{2 \times 60}}{{15}}\,{\text{kmph}} \cr & = 8\,{\text{kmph}} \cr} $$
Rate downstream of boat = 5 kmph
Speed of boat in still water = $$\frac{1}{2}$$ (Rate downstream + Rate upstream)
$$\eqalign{ & = \frac{1}{2}\left( {8 + 5} \right) \cr & = \frac{{13}}{2} \cr & = 6\frac{1}{2}\,{\text{kmph}} \cr} $$
10. A boat takes half time in moving a certain distance downstream than upstream. The ratio of the speed of the boat in still water and that of the current is?
a) 2 : 1
b) 4 : 3
c) 1 : 2
d) 3 : 1
Discussion
Explanation: Let the speed of boat in still water = x km/hr,
and Speed of current = y km/hr
Rate downstream = (x + y) km/hr, and Rate upstream = (x – y) km/hr
Distance = Speed × Time
$$\eqalign{ & \left( {x - y} \right) \times 2t = \left( {x + y} \right) \times t \cr & 2x - 2y = x + y \cr & 2x - x = 2y + y \cr & x = 3y \cr & \Rightarrow \frac{x}{y} = \frac{3}{1} = 3:1 \cr} $$
11. A boat can travel with a speed of 13 km/hr in still water. If the speed of the stream is 4 km/hr, find the time taken by the boat to go 68 km downstream.
a) 2 hours
b) 3 hours
c) 4 hours
d) 5 hours
Discussion
Explanation: Speed downstream
= (13 + 4) km/hr
= 17 km/hr
Time taken to travel 68 km downstream
= $$\frac{{68}}{{17}}$$ hours
= 4 hours
12. The ages of Shakti and Kanti are in the ratio of 8 : 7 respectively. After 10 years, the ratio of their ages will be 13 : 12. What is the difference between their ages ?
a) 8.5 km/hr
b) 9 km/hr
c) 10 km/hr
d) 12.5 km/hr
Discussion
Explanation: Man's rate in still water
= (15 - 2.5) km/hr
= 12.5 km/hr.
Man's rate against the current
= (12.5 - 2.5) km/hr
= 10 km/hr.
13. A boat running upstream takes 8 hours 48 minutes to cover a certain distance, while it takes 4 hours to cover the same distance running downstream. What is the ratio between the speed of the boat and speed of the water current respectively?
a) 2 : 1
b) 3 : 2
c) 8 : 3
d) None of these
Discussion
Explanation: Let the man's rate upstream be x kmph and that downstream be y kmph.
Then, distance covered upstream in 8 hrs 48 min = Distance covered downstream in 4 hrs.
$$\eqalign{ & {x \times 8\frac{4}{5}} = {y \times 4} \cr & \frac{{44}}{5}x = 4y \cr & y = \frac{{11}}{5}x \cr & {\text{Required}}\,{\text{ratio}} \cr & = {\frac{{y + x}}{2}} : {\frac{{y - x}}{2}} \cr & = \left( {\frac{{16x}}{5} \times \frac{1}{2}} \right):\left( {\frac{{6x}}{5} \times \frac{1}{2}} \right) \cr & = \frac{8}{5}:\frac{3}{5} \cr & = 8:3 \cr} $$
14. A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:
a) 4 km/hr
b) 5 km/hr
c) 6 km/hr
d) 10 km/hr
Discussion
Explanation: Let the speed of the stream be x km/hr
Speed downstream = (15 + x) km/hr
Speed upstream = (15 - x) km/hr
$$\eqalign{ & \frac{{30}}{{ {15 + x} }} + \frac{{30}}{{ {15 - x} }} = 4\frac{1}{2} \cr & \frac{{900}}{{225 - {x^2}}} = \frac{9}{2} \cr & 9{x^2} = 225 \cr & {x^2} = 25 \cr & x = 5\,km/hr \cr} $$
15. In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
a) 3 kmph
b) 5 kmph
c) 8 kmph
d) 9 kmphr
Discussion
Explanation: Speed in still water
= $$\frac{1}{2}$$(11 + 5) kmph
= 8 kmph.
16. A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?
a) 40 minutes
b) 1 hour
c) 1 hr 15 min
d) 1 hr 30 min
Discussion
Explanation:
$$\eqalign{ & {\text{Rate}}\,{\text{downstream}} \cr & = \left( {\frac{1}{{10}} \times 60} \right)km/hr = 6\,km/hr \cr & {\text{Rate}}\,{\text{upstream}} = 2\,km/hr \cr & {\text{Speed}}\,{\text{in}}\,{\text{still}}\,{\text{water}} \cr & = \frac{1}{2}\left( {6 + 2} \right)km/hr = 4\,km/hr \cr & {\text{Required}}\,{\text{time}} = {\frac{5}{4}} \,hrs \cr & = 1\frac{1}{4}hrs \cr & = 1\,hr\,15\,\min . \cr} $$
17. A man can row three-quarters of a kilometer against the stream in $$11\frac{1}{4}$$ minutes and down the stream in $$7\frac{1}{2}$$ minutes. The speed (in km/hr) of the man in still water is:
a) 2
b) 3
c) 4
d) 5
Discussion
Explanation: We can write three - quarters of a kilometer as 750 meters and $$11\frac{1}{4}$$ minutes as 675 seconds
$$\eqalign{ & {\text{Rate}}\,{\text{upstream}} \cr & = {\frac{{750}}{{675}}} m/\sec = \frac{{10}}{9}m/\sec \cr & {\text{Rate}}\,{\text{downstream}} \cr & = {\frac{{750}}{{450}}} m/\sec = \frac{5}{3}m/\sec \cr & {\text{Rate}}\,{\text{in}}\,{\text{still}}\,{\text{water}} = \frac{1}{2}\left( {\frac{{10}}{9} + \frac{5}{3}} \right)m/\sec \cr & = \frac{{25}}{{18}}\,m/\sec \cr & = \left( {\frac{{25}}{{18}} \times \frac{{18}}{5}} \right)km/hr \cr & = 5\,km/hr \cr} $$
18. Speed of a boat in standing water is 9 kmph and the speed of the stream is 1.5 kmph. A man rows to a place at a distance of 105 km and comes back to the starting point. The total time taken by him is:
a) 16 hours
b) 18 hours
c) 20 hours
d) 24 hours
Discussion
Explanation: Speed upstream = 7.5 kmph
Speed downstream = 10.5 kmph
Total time taken
= $$\left( {\frac{{105}}{{7.5}} + \frac{{105}}{{10.5}}} \right)$$ hours
= 24 hours
19. A man takes twice as long to row a distance against the stream as to row the same distance in favour of the stream. The ratio of the speed of the boat (in still water) and the stream is:
a) 2 : 1
b) 3 : 1
c) 3 : 2
d) 4 : 3
Discussion
Explanation: Let man's rate upstream be x kmph
Then, his rate downstream = 2x kmph
(speed in still water) : (Speed of stream)
$$\eqalign{ & = {\frac{{2x + x}}{2}} : {\frac{{2x - x}}{2}} \cr & = \frac{{3x}}{2}:\frac{x}{2} \cr & = 3:1 \cr} $$.
20. A man rows to a place 48 km distant and come back in 14 hours. He finds that he can row 4 km with the stream in the same time as 3 km against the stream. The rate of the stream is:
a) 1 km/hr
b) 1.5 km/hr
c) 2 km/hr
d) 2.5 km/hr
Discussion
Explanation: Suppose he move 4 km downstream in x hours
Speed downstream = $$\frac{4}{x}$$ km/hr
Speed upstream = $$\frac{3}{x}$$ km/hr
$$\eqalign{ & \therefore \frac{{48}}{{\left( {4/x} \right)}} + \frac{{48}}{{\left( {3/x} \right)}} = 14\,or\,x = \frac{1}{2} \cr} $$
So, Speed downstream = 8 km/hr
Speed upstream = 6 km/hr
Rate of the stream = $$\frac{1}{2}$$(8 - 6) km/hr
= 1 km/hr
21. A man can row at a speed of $$4\frac{1}{2}$$ km/hr in still water. If he takes 2 times as long to row a distance upstream as to row the same distance downstream, then the speed of stream (in km/hr) is-
a) 1
b) 1.5
c) 2
d) 2.5
Discussion
Explanation:
$$\eqalign{ & {\text{Speed of man in still water , }} \cr & x{\text{ }} = {\text{ }}\frac{9}{2}km/hr{\text{ }} \cr & {\text{let speed of stream = }}y{\text{ }}km/h \cr & {\text{Downstream speed = }} {\frac{9}{2} + y} \cr & {\text{Upstream speed = }} {\frac{9}{2} - y} \cr & {\text{Accroding to questions,}} \cr & {\text{2}} \times {\frac{{{\text{Distance}}}}{{ {\frac{9}{2} + y} }}} = \frac{{{\text{Distance}}}}{{ {\frac{9}{2} - y} }} \cr & \frac{2}{{\frac{9}{2} + y}} = \frac{1}{{\frac{9}{2} - y}} \cr & \frac{{2 \times 2}}{{9 + 2y}} = \frac{2}{{9 - 2y}} \cr & \frac{2}{{9 + 2y}} = \frac{1}{{9 - 2y}} \cr & 18 - 4y = 9 + 2y \cr & 6y = 9 \cr & \Rightarrow y = \frac{9}{6} = \frac{3}{2} = 1.5\,km/h \cr} $$
22. A boat covers 24 km upstream and 36 km downstream in 6 hours, while it covers 36 km upstream and 24 km downstream in $$6\frac{1}{2}$$ hours. The speed of the current is ?
a) 1 km/hr
b) 2 km/hrs
c) 1.5 km/hrs
d) 2.5 km/hr
Discussion
Explanation: let speed of boat in still water = x km/h
Speed of stream current = y km/h
$$\eqalign{ & \frac{{24}}{{x - y}} + \frac{{36}}{{x + y}} = 6h\,......\,(i) \cr & \frac{{36}}{{x - y}} + \frac{{24}}{{x + y}} = \frac{{13}}{2}h\,......\,(ii) \cr} $$
In these type of questions, make factor of 24 and 36 and choose the common values which satisfy the above equations.
$$\eqalign{ & {\text{24 = 2,3,4,6,8,}}\boxed{12} \cr & 36 = 3,4,9,\boxed{12} \cr} $$
Choose the common factor i.e. Put this value in equation (i)
$$\eqalign{ & \frac{{24}}{{x - y}} + \frac{{36}}{{12}} = 6 \cr & \frac{{24}}{{x - y}} + 3 = 6 \cr & x - y = 8 \cr & x + y = 12 \cr & x = 10\,\,\,,\,\,\,\,y = 2 \cr & {\text{Speed of the current,}} \cr & y = 2{\text{ km/h}} \cr} $$
23. A boat while dowenstream in a reiver converd a distance of 50 miles at an average speed of 60 miles per hour. While returning , because of the water resistance , it took 1 hour 15 minutes to cover the same distance What was the average speed during the whole journey?
a) 40 mph
b) 48 mph
c) 50 mph
d) 55 mph
Discussion
Explanation:
$$\eqalign{ & {\text{Time taken to cover 50 miles downstream}} \cr & {\text{ = }}\left( {\frac{{50}}{{60}}} \right)hr{\text{ = }}\frac{5}{6}hr.{\text{ }} \cr & {\text{Time taken to cover 50 miles upstream}} \cr & {\text{ = 1hr 15m = 1}}\frac{1}{4}hrs = \frac{5}{4}hrs \cr & {\text{Total time taken to cover 100 miles}} \cr & {\text{ = }}\left( {\frac{5}{6} + \frac{5}{4}} \right)hrs = \frac{{25}}{{12}}hrs \cr & {\text{Average speed }} \cr & {\text{ = }}\frac{{100}}{{\left( {\frac{{25}}{{12}}} \right)}}mph \cr & = \left( {\frac{{100 \times 12}}{{25}}} \right)mph \cr & = 48mph. \cr} $$
24. A boat travels 24 km upstream in 6 hours and 20 km downstream in 4 hours. Then the speed of boat in still water and the speed of current are respectively
a) 4 kmph and 3 kmph
b) 4.5 kmph and 0.5 kmph
c) 4 kmph and 2 kmph
d) 5 kmph and 2 kmph
Discussion
Explanation:
$$\eqalign{ & {\text{Upstream speed, U}} \cr & {\text{ = }}\frac{{24}}{6} = 4\,km/h \cr & {\text{Downstream speed , D}} \cr & {\text{ = }}\frac{{20}}{4} = 5\,km/h \cr & {\text{Speed of boat in still water , }}x \cr & = \frac{{{\text{D + U}}}}{2} = \frac{9}{2} = 4.5\,km/h \cr & {\text{Speed of water current, }}y \cr & = \frac{{{\text{D - U}}}}{2} = \frac{1}{2} = 0.5\,km/h \cr} $$
25. A man can row 6 km/hr in still water. If the speed of the current is 2 km/hr he takes 4 hours more in upstream than the downstream. The distance is?
a) 30 km
b) 24 km
c) 20 km
d) 32 km
Discussion
Explanation:
Speed of man in still water, x = 6 km/h
Speed of current, y = 2 km/h
Let distance = M
Upstream time = Downstream time + 4
$$\eqalign{ & \frac{M}{4} = \frac{M}{8} + 4 \cr & \frac{M}{4} = \frac{{M + 32}}{8} \cr & M = 32 \cr & {\text{Distance = 32 }}km \cr} $$
26. If the speed of a boat in still water is 20km/hr and the speed of the current is 5km, then the time taken by the boat to travel 100 km with the current is?
a) 2 hours
b) 3 hours
c) 4 hours
d) 7 hours
Discussion
Explanation: Relative speed = 20 + 5 = 25 km/hr
Time = $$\frac{{100}}{{25}}$$ = 4 hours
27. A man takes 2.2 times as long to row a distance upstream as to row the same distance downstream. If he can row 55 km downstream in 2 hour 30 minutes, what is the speed of the boat in the still water?
a) 40 km/h
b) 8 km/h
c) 16 km/h
d) 24 km/h
Discussion
Explanation:
$$\eqalign{ & {\text{Speed of the boat in downstream }} \cr & {\text{ = }}\frac{{55}}{{2.5}} = \frac{{55 \times 10}}{{25}} = 22\,km/hr \cr & {\text{Then, speed of the boat in upstream}} \cr & {\text{ = }}\frac{{22}}{{2.2}} = \frac{{22 \times 10}}{{22}} = 10\,km/hr \cr & {\text{Speed of the boat in still water}} \cr & {\text{ = }}\frac{{22 + 10}}{2} = 16\,km/hr \cr} $$
28. The speed of the boat in still water is 5 times that of current, it takes 1.1 hour to row to point B from point A downstream. The distance between point A and point B is 13.2 km. How much distance (in km) will it cover in 312 minutes upstrem?
a) 43.2
b) 48
c) 41.6
d) 44.8
Discussion
Explanation: Let the speed of the current be x kmph
Then speed of the boat in still water = 5x
$$\eqalign{ & {\text{Downstream speed}} \cr & {\text{ = }}\left( {5x + x} \right) = 6x\,kmph \cr & {\text{Upstream speed}} \cr & {\text{ = }}\left( {5x - x} \right) = 4x\,kmph \cr & {\text{Now, }} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{13}}{\text{.2km}}\,\,\,\, \cr & {\text{A}}\overline {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} {\text{B}} \cr & {\text{According to question,}} \cr & {\text{1}}{\text{.1}} \times {\text{6}}x = 13.2 \cr & \Rightarrow 6.6x = 13.2 \cr & \Rightarrow x = \frac{{13.2}}{{6.6}} \cr & x = 2\,kmph \cr & {\text{Upstream speed}} \cr & {\text{ = 4}}x = 4 \times 2 = 8\,kmph \cr & {\text{312 minutes}}\, \cr & = 5\frac{1}{5}\,hours \cr & = \frac{{26}}{5}\,hours \cr & {\text{Required distance travelled upstream}} \cr & {\text{ = Speed }} \times {\text{Time}} \cr & {\text{ = 8}} \times \frac{{26}}{5} = 41.6\,km \cr} $$
29. A boat can tarvel 36 km upstream in 5 hours. If the speed of the stream is 2.4 kmph, how much time will the boat take to cover a distance of 78 km downstream?(in hours)
a) 5 hours
b) 6.5 hours
c) 5.5 hours
d) 8 hours
Discussion
Explanation: Distance covered by a boat in 5 hours = 36 km
Rate upstream of boat = $$\frac{{36}}{5}$$ = 7.2 kmph
Speed of the stream = 2.4 kmph
Speed of the boat in still water
= (7.2 + 2.4) kmph
= 9.6 kmph
Rate downstream of the boat
= (9.6 + 2.4) kmph
= 12 kmph
Time taken in covering 78 km distance
= $$\frac{{78}}{{12}}$$
= 6.5 hours
30. A man can row upstream at 12km/hr and downstream at 18 km/hr. The man rowing speed in still water is?
a) 15 km/hr
b) 5 km/hr
c) 25 km/h
d) 10 km/h
Discussion
Explanation: Speed of boat in still water = $$\frac{{x + y}}{2}$$
Where (x = downstream speed) and (y = upstream speed)
Boat's speed
$$\eqalign{ & {\text{ = }}\frac{{18 + 12}}{2} \cr & = \frac{{30}}{2} \cr & = 15\,km/hr \cr} $$