A measuring cup contains 1/5 of a cup of orange juice.It is then filled to the 1 cup mark with a mix

题目

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,grapefurit,and pineapple juices.What fraction of the mixture is orange juice?答案是7/15
为什么是4/5呢？第二个代数式解释下吧~

答案

（1/5）+（4/5）*（1/3）=7/15
还需要细致解释不.
原先那个杯子里已经有1/5的橘子汁了；然后题目说"filled to 1 cup"就是说倒满,那后面混合果汁就占杯子容量的4/5,混合果汁里有3中水果,每种各占1/3,所以混合果汁的橘子汁占杯子总容量的4/5*1/3=4/15