A boat goes 21 km downstream in the same time as it goes 15 km upstream. The stream flows at 2 km/h. What is the boat's speed in still water?

Explanation

1. 01

   Equal times: $\frac{21}{b + 2} = \frac{15}{b - 2}$.

2. 02

   $21b - 42 = 15b + 30$, so $6b = 72$ and $b = 12$ km/h.