A boat goes 30 km upstream in 3 hours and downstream in 1 hour. How much time (in hours) will this boat take to cover 60 km in still water?

3

If we let the speed of the boat = a and the speed of the stream = b then,

Downstream = a + b

Upstream = a – b

According to the question

A boat goes 30 km upstream in 3 hours 

= (a – b) = \(\frac{30}{3}\)

= a – b = 10 .....(1)

Again,

A boat goes 30 km downstream in 1 hour

= a + b = 30 .....(2)

Adding the equation (1) and (2), we get

= a – b = 10 + a + b = 30

= 2a = 40

= a = 20

Now,

Time taken to cover 60 km in still water \(\frac{60}{20}\) = 3