If the ratio of upstream speed to downstream speed is $3:5$ and the still-water speed is $24$ km/h, then the total time to travel $36$ km upstream and $60$ km downstream is:

Question

If the ratio of upstream speed to downstream speed is $3:5$ and the still-water speed is $24$ km/h, then the total time to travel $36$ km upstream and $60$ km downstream is:

Option A

3 hours

Option B — Correct answer

4 hours

Correct choice.

Option C

4.5 hours

Option D

5 hours

Explanation

Let upstream and downstream speeds be $3x$ and $5x$. Their average gives still-water speed: $\frac{3x+5x}{2}=24$, so $x=6$. Upstream speed $=18$ and downstream speed $=30$. Total time $=36\div18+60\div30=2+2=4$ hours.

Chapter Mock 9: Boat and Stream