A boat can tavel 10 km upstream and 20 km downstream in 7 hours. It can travel 20 km upstream and 10 km downstream in 11 hours. What is the speed of bout in still water?

6 km/h

Let the speed of the boat = a and speed of the stream = b

Given,

A boat can tavel 10 km upstream and 20 km downstream in 7 hours. It can travel 20 km upstream and 10 km downstream in 11 hours.

According to the question,

\(\frac{10}{a-b}\) + \(\frac{20}{a+b}\) = 7 ----(M)

\(\frac{20}{a-b}\) + \(\frac{10}{a+b}\) = 11 ----(N)

By multiplying equation M with 2 we get,

\(\frac{30}{a+b}\) = 3

a + b = 10-----(k)

By multiplying equation N with 2 we get,

\(\frac{-30}{a-b}\) = 15

a - b = 2-----(l)

So, from k and l

a = 6 km/h