A boat moves at 10 km/h in still water. It goes 48 km downstream and comes back in 10 hours in all. What is the speed of the stream?

Explanation

1. 01

   $\frac{48}{10 + s} + \frac{48}{10 - s} = 10$, so $\frac{960}{100 - s^2} = 10$.

2. 02

   $100 - s^2 = 96$, so $s = 2$ km/h. Check: $4 + 6 = 10$ hours.