Length of a river from drifting and swimming times

A river has an unknown length and an unknown, constant current. Floating without swimming, the current carries you from one end to the other in $6$ seconds. Swimming with the current at $3$ feet per second relative to the water, you cover the same distance in $4$ seconds. How long is the river, in feet?

$36$ feet. With current $v$ and length $L$: $L=6v$ and $L=4(v+3)$. So $6v=4v+12$, $v=6$ ft/s and $L=36$.