if the ratio of fruits to vegetables is 5:7 then we multiply something on both sides to get the side that we know, and the other side is our answer. So if monkey A receives 30 fruits, we multiply 6 on both sides to get 30:42, so monkey A receives 42 vegetables. for monkey B he receives 70 vegetables, so we multiply 10 on both sides, to get 50:70, so monkey B receives 50 fruits.
