A speedboat, whose speed in 20 km/h in still water, goes 35 km downstream and comes back in a total of 8 hours. What is the speed of the stream?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. 15 km/h.
Given that:
The speed of a boat in still water = 20 km/h.
Let the speed of stream be y km/h.
So,
Downstream speed = 20 + y
Upstream speed = 20 - y
According to the question:
Total time to go 35 km downstream and come back = 8 hrs
⇒ 35/(20 + y) + 35/(20 - y) = 8
⇒ 35 (20 - y + 20 + y) = 8 (400 - y2)
⇒ 35 × 40 = 8 (400 - y2)
⇒ 400 - y2 = 175
⇒ y2 = 400 - 175 = 225
⇒ y = 15 km/h
Hence,
Speed of stream = 15 km/hr