SOLUTION: John mixes a certain amount of 10% isopropyl mixture with 60 gallons of a 20% isopropyl solution, and the resulting mixture is 16% isopropyl. How much of the 10% isopropyl did he

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In words:
(gallons of isopropyl) / (gallons of mixture) = 16%
Let = amount of 10% mixture to be added
gallons = amount of 20% mixture added
The total gallons of mixture =


multiply each sides by




He added 40 gallons of the isopropyl mixture
check:





OK