2021 · Q33
A motor boat has speed 30 km/hr in still water. It goes 60 km down stream and comes back in 9/2 hours. What is the speed of the stream ?
Elementary MathematicsTime, Speed and Distance
- A5 km/hr
- B8 km/hr
- C10 km/hr
- D12 km/hr
Explanation
Let the stream speed be v km/hr. Then 60/(30+v) + 60/(30−v) = 9/2. Combining: 3600/(900 − v²) = 9/2, so 900 − v² = 800 and v² = 100. Hence v = 10 km/hr, option (c).