2023 · Q40
The speed of a boat in still water is 15 km/hr. If it can travel 42 km downstream and 28 km upstream in the same time, then what is the speed of the stream?
Elementary MathematicsTime, Speed and Distance
- A2.5 km/hr
- B3 km/hr
- C4.5 km/hr
- D6 km/hr
Explanation
Equal travel times give 42/(15 + v) = 28/(15 − v). Cross-multiplying: 42(15 − v) = 28(15 + v), i.e. 630 − 42v = 420 + 28v. Hence 70v = 210 and v = 3 km/h (B).