The speed of a boat in still water is 10 km/h. If its speed downstream is 18 km/h, the distance travelled by the boat upstream in 5 hours is :

10 km

The correct answer is Option (2) → 10 km

Let the speed of the stream be $x$ km/hr

Downstream speed = $10+x=18$

$⇒x=8km/hr$

Upstream speed = $10-8=2km/hr$

Distance = Speed × Time

$=2×5=10km$