The time taken by a motor boat moving downstream in a river to cover a distance and the time taken by the motor boat to cover the same distance upstream are in the ratio 3:5. If the speed of the stream is 10 km/hr, what is the speed of the boat?
A20 km/hr
B25 km/hr
C30 km/hr
D40 km/hr ✓ Correct
Correct answer: (D) 40 km/hr
Explanation
The speed of the boat is 40 km/hr.
Time ratio downstream to upstream is 3:5, so speed ratio is 5:3.
So (b + 10)/(b - 10) = 5/3.
3(b + 10) = 5(b - 10) gives 3b + 30 = 5b - 50.
2b = 80, so b = 40 km/hr.
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