A boat can travel 20 km downstream in 24 min. The ratio of the speed of the boat in still water to the speed of the stream is 4 : 1. How much time will the boat take to cover 15 km upstream?

Question: A boat can travel 20 km downstream in 24 min. The ratio of the speed of the boat in still water to the speed of the stream is 4 : 1. How much time will the boat take to cover 15 km upstream?

Solution:
Downstream speed = {(20/24) × 60} km/hr
= 50 km/hr

Let,
speed of water and stream is 4a and a.
Downstream speed = (4a + a) = 5a
and upstream speed = (4a - a) = 3a

ATQ,
5a = 50
∴ a = 10
So, upstream speed = (3 × 10) = 30 km/hr.
∴ Time taken to go 15 km upstream = (15 × 60)/30 = 30 min.