A motorboat whose speed in still water is 15 km/h goes 30 km downstream and returns in a total time of 4.5 hours. What is the speed of the stream?

Explanation

Let stream speed be v. 30/(15+v) + 30/(15-v) = 4.5 => 30[(15-v + 15+v)/(225 – v^2)] = 4.5 => 30[30/(225 – v^2)] = 4.5 => 900 = 4.5(225 – v^2) => 200 = 225 – v^2 => v^2 = 25 => v = 5 km/h.