Correct Answer - Option 4 : 8 km
Given:
The boat can travel 780 km downstream in 10 h
Formula used:
Downstream speed = Speed in still water + Speed of stream
Calculation:
Let the speed of the boat in still water be x and the speed of the stream be y.
780/(x + y) = 10
⇒ x + y = 78
⇒ y = 38
⇒ x = 40
Upstream speed = 40 - 38 = 2 km/h
Distance in 4 hours = 2 × 4 = 8 km
∴ The required distance covered is 8 km.
