A boat moves at 15 km/h in still water. It goes 30 km downstream and comes back in 4 hours 30 minutes in all. What is the speed of the stream?

Explanation

1. 01

   $\frac{30}{15 + s} + \frac{30}{15 - s} = 4.5$, which gives $\frac{900}{225 - s^{2}} = 4.5$.

2. 02

   $225 - s^{2} = 200$, so $s^{2} = 25$ and $s = 5$ km/h.