$4$ people are standing by a river. There is only $1$ boat, and it can only seat $2$ people at one time. The $4$ of them take $7$ minutes, $9$ minutes, $4$ minutes and $10$ minutes respectively to row the boat across the river. What is the shortest period of time they will take to cross the river? 
Overseas Competition->Knowledge Point->Combinatorics->Fun Problems in Math->Fun Math Problems->Strategies for Crossing River Problems