a) 15 km/hr
b) 5 km/hr
c) 25 km/h
d) 10 km/h
Answer: a
Explanation: Speed of boat in still water = $$\frac{{x + y}}{2}$$
Where (x = downstream speed) and (y = upstream speed)
Boat’s speed
$$\eqalign{
& {\text{ = }}\frac{{18 + 12}}{2} \cr
& = \frac{{30}}{2} \cr
& = 15\,km/hr \cr} $$
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