a) 2 hrs
b) 4 hrs
c) 2.4 hr
d) 1.2 hrs
Answer: c
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
$$\eqalign{
& = \frac{{24\,km}}{{10\,km/hr}} \cr
& = 2.4\,{\text{hours}} \cr} $$
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