A rowing crew covers 33 km down a river in 3 hours and 20 km up the same river in 4 hours. Find the speed (in km/h) of the current.

Let the crew's still-water speed be $x$ km/h and the current's speed $y$ km/h. $x+y=33/3=11$ and $x-y=20/4=5$. Subtracting: $2y=6$, so $y=3$ and $x=8$. Check: 8 + 3 = 11 and 8 - 3 = 5. 8 is the crew's speed in still water, the other unknown; 11 and 5 are the downstream and upstream speeds, not the current itself.