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Ratio of Three Variables Example

To make a special beverage, the water, apple juice, and lemon juice are mixed by volume in a ratio of 3 : 4 : 5 respectively. In order to make 9 gallons of this beverage, how many gallons of apple juice is needed ?

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.

what is ratio and how to solve three-variable ratio?