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A boat takes 3 hours less time in rowing from point X to point Y in downstream than that in rowing from point Y to point Z in upstream. The distance between X and Y is 20 km which is half of the distance between Y and Z. Speed of boat B in still water is 10 kmph. The speed of boat A in still water is equal to the rate upstream of boat B. What is the speed of boat A in still water? (in kmph).

A) 12
B) 8
C) 10
D) 11

Correct Answer:


B) 8

Description for Correct answer:
Speed of current = x kmph (let)

Rate Upstream of boat B = (10 - x) kmph

Rate downstream of boat A

= 10 - x + x = 10 kmph

\( \Large \therefore \frac{40}{10 - x} - \frac{20}{10} = 3 \)

=> \( \Large \frac{40}{10 - x} = 5 \)

=> 50 - 5x = 40

=> 5x = 50 - 40 = 10

=> \( \Large x = \frac{10}{5} = 2 \ kmph \)

\( \Large \therefore \)Speed of boat A in still water = 10 - 2 = 8 kmph

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