What is the speed of the boat in still water?

Aug 2014
504
1
India
A boat goes 24 km upstream and 36 km downstream in 8 hr 30 min, while if it goes 36 km upstream and 24 km downstream, the boat takes 9 hours. What is the speed of the boat in still water?

A) 24 km (Upstream) + 36 km (Downstream) → 8 hrs 30 min → 1
36 km (Upstream) + 24 km (Downstream) → 9 hrs → 2

1.5 * 2

9hrs*1.5 = 13.5 hr i.e 13 hr 30 min

54 km (Upstream) + 36 km (Downstream) → 13 hr 30min → 2
30 km (Upstream) → 5 hrs.

→ Upstream speed = 30/5 = 6 kmph

From here How to proceed?
 

skipjack

Forum Staff
Dec 2006
21,184
2,332
Using 1.5 * 1 instead of 1.5 * 2, you can find that the downstream speed is 8 kmph. Alternatively, you can substitute the upstream value into 1 to find the downstream speed.

The speed of the boat in still water is the arithmetic mean of its upstream and downstream speeds, which is 7 kmph.