Canoeing
Saw this problem from J. A. H. Hunter at the Futility Closet.
http://www.futilitycloset.com/2015/05/17/a-passing-wave/
I generalized it a little, in hopes that students would see it less as a trick and more as a structure. I love these problems that seem to have too little information, but students are often stumped where to start. I'm hoping an interactive would let them play with it more.
The sketch randomly figures out paddling time, the speed of the river current, and the speed of the paddlers. Is that really enough information to figure out the speed of the river?