Jason, Chi Yew, Wei Jie and Michael are on their way home. There is only $1$ boat, and it can only seat $2$ people at one time. Jason takes $5$ minutes to cross the river, Chi Yew takes $5$ minutes, Wei Jie takes $9$ minutes and Michael takes $7$ minutes.  They try to find the shortest period of time they will take to cross the river using the following working:  $4\times5$ min $=20$ min  Are they correct? If not, show the correct working and answer. 
Overseas Competition->Knowledge Point->Combinatorics->Fun Problems in Math->Fun Math Problems->Strategies for Crossing River Problems