A measuring cup contains 1/5 of a cup of orange juice.It is

The mixture is 1 - 1/5 = 4/5 of the cup,containing equal amounts of orange,grapefruit,and pineapple juices,so in this part,each juice is (4/5)*(1/3) = 4/15 of the cup; orange juice is 4/15 + 1/5 = 7/15 of the cup,therefore:
Orange juice:7/15
Grapefruit juice:4/15
pineapple juice:4/15