As water, apple juice and lemon juice mixed together in the ratio of 3 : 4 : 5 , the water makes up 3/(3+4+5) of the mix, apple juice makes up 4/(3+4+5) of the mix, and lemon juice makes up 5/(3+4+5) of the mix. Since apple juice makes up 4/12 of the mix, then 9 gallons of mix need x gallons of apple juice.