A boat has a speed of 12 km/hr in still water, and the stream flows at 4 km/hr. The boat goes a certain distance downstream and returns the same distance upstream. What is its average speed for the whole journey?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(10\frac{2}{3}\) 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 12 km/hr.
Downstream speed \(=12+4=16\) km/hr, upstream speed \(=12-4=8\) km/hr
Take each leg as 16 km (any distance gives the same answer)
Time \(\displaystyle =\frac{16}{16}+\frac{16}{8}=1+2=3\) hours
Average speed \(\displaystyle =\frac{16+16}{3}=\frac{32}{3}=10\tfrac{2}{3}\) km/hr