一道AMC的数学题,谁能帮忙解决一下
5.Atanelementaryschool,thestudentsinthirdgrade,fourthgrade,andfifthgraderunanaverageof12,15,and10minutesperday,respectively.Therearetwiceasmanythirdgradersasfourthgraders,andtwiceasmanyfourthgradersasfifthgraders.Whatistheaveragenumberofminutesrunperdaybythesestudents?