都参加的有4人,说明重复了4个人,所以12+8-4=16就可以了
http://zhidao.baidu.com/question/1604769103566818987.html?istorch=1&torchtoken=08c2Z7forIdePDrFbB7YhVziSeSmnqmtP4+osotDlV82OfTiBqmneEUrmsXacP70QB6wy6byhqHFI0wSxpZDQXGCCPI+RlNypeWbNf1s++c7Nt2AmZIl