A boat goes 10 km upstream in 50 minutes, and the speed of the stream is 3 kmph. Find the speed of the boat in still water (in km/h).

Question: A boat goes 10 km upstream in 50 minutes, and the speed of the stream is 3 kmph. Find the speed of the boat in still water (in km/h).

Solution:
Let the speed of the boat in still water = x km/h
Speed of the stream = 3 km/h (given)
Upstream speed = x - 3 km/h
Distance upstream = 10 km
Time upstream = 50 minutes = 50/60 hours = 5/6 hours

We know,
Speed = Distance/Time
Upstream speed = 10 /(5/6) = 10 × (6/5) = 12 km/h
So,
⇒ x - 3 = 12
⇒ x = 12 + 3
∴ x = 15 km/h

The speed of the boat in still water is 15 km/h.