一道sat数学题

题目

一道sat数学题
a measuring cup contains 1/5 of a cup of orange juice .it is then filled to the 1 cup mark with a mixture that contains equal amounts of orange,grapefruit,and pinapple juices.what fraction of the final mixture is orange juice 答案是7/15 怎么算的?

答案

很简单啊,原来杯子里面是1/5的orange juice
后来杯子被加满,加入的是三种混合起来的果汁,橙汁占其中的1/3
最后橙汁的比例：
原来的1/5+ 后来加入的（1-1/5）* 1/3
=1/5+ 4/15
=7/15