问下这道gre数学
问题描述：
问下这道gre数学
how many qarts of pure alcohol must you add to 15 quarts of solution that is 40% alcohol to strengthen it to a solution that is 50% alcohol?
答
现有40%酒精15夸脱,你加入多少纯酒精才能让液体浓度达到50%?
设你要加入的为x夸脱,则
15*40% + x*100% = (15 + x)*50%
解得 x = 3