A boat can travel 21 km in 3 hours going upstream. It can travel 55 km in 5 hours going downstream. What is the speed of the stream?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 2 km/h
Let the speed of the boat in still water be b, and the speed of the stream be s.
When the boat travels upstream, its effective speed is (b - s). Therefore, we can write:
(b - s) × 3 = 21
Simplifying, we get:
b - s = 7 ...(1)
When the boat travels downstream, its effective speed is (b + s). Therefore, we can write:
(b + s) × 5 = 55
Simplifying, we get:
b + s = 11 ...(2)
Adding equations (1) and (2), we get:
2b = 18
Therefore, b = 9.
Substituting this value of b in equation (1), we get:
9 - s = 7
Therefore, s = 2.
The speed of the stream is 2 km/h.