A boat goes 20 km downstream in 30 mins. The speed of the stream is 10 km/h. Find the speed of the boat.

Correct option is D

Given:
Distance travelled in downstream = 20 km
Time taken = 30 min = 0.5 hr
Speed of stream = 10 km/h
Formula Used:
Speed of boat in downstream = Speed of boat in Still water + Speed of River
Speed = Distance/Time
Solution:
Speed of downstream = $$\frac{20}{0.5}$$​ = 40 km/hr
Speed of boat in downstream = Speed of boat in Still water + Speed of River
=> Speed of boat in Still water + 10 = 40
=> Speed of boat in Still water = 40 - 10
=> Speed of boat in Still water = 30 km/h
∴ Speed of boat is 30 km/h.