A tourist boat moves at 18 km/hr in still water, and the river flows at 6 km/hr. The boat goes from a ghat up the river to an island and comes back to the ghat by the same route. What is its average speed for the whole trip?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 16 km/hr
Average speed is total distance divided by total time. The boat spends longer on the slow upstream leg, so the average is less than 18 km/hr.
Upstream speed \(=18-6=12\) km/hr, downstream speed \(=18+6=24\) km/hr
Take each leg as 24 km (any distance gives the same answer)
Time \(\displaystyle =\frac{24}{12}+\frac{24}{24}=2+1=3\) hours
Average speed \(\displaystyle =\frac{24+24}{3}=\frac{48}{3}=16\) km/hr