A boat's crew rowed 15 kilometers downstream, with the current, in 1 hours. The return trip upstream, against the current, covered the same distance, and it took 3 hours. Find the crew's rowing rate in still water.

B and c are the speeds of the boat (in still water) and the current.

Traveling with the current, the boat goes b+c km per hour.
1 hour * (b+c km)/hour = 15 km
b+c = 15

Traveling against the current, the boat goes b-c km per hour.
3 hours * (b-c km)/hour = 15
b-c = 5

Add the two equations
b = 10 km per hour