男同学比女同学多：（30-20）÷20=50%
男同学人数减女同学人数:30-20=10 10÷20×100％=50％
(30-20)÷20×100%=50%男同学比女同学多50%
多=(30-20)÷20=50%
(30-20)÷20x100%=50%
