A boat covers 4 km against the stream in 1 hour and covers the same distance in the direction of stream in 40 minutes. How long will it take to go 10 km in still water ?

2 hours

The correct answer is option (1) : 2 hours

Upstream speed $v=\frac{4}{1}= 4\, km/h$

Downstream speed $u =\frac{4}{\frac{40}{60}}=6\, km/h$

∴ Speed of boat in still water $=\frac{u+v}{2}$

$=\frac{4+6}{2}$

$= 5\, km/h$

So, the time taken by boat to cover the distance of 10 km in still water $=\frac{10}{5}= 2hours $