How many gallons of a 90% antifreeze solution must be mixed with 90 gallons of 20% antifreeze to get a mixture that is 80% antifreeze?

540

Step-by-step explanation:

Amount of antifreeze in the 90% solution + amount of antifreeze in the 20% solution = amount of antifreeze in the 80% solution

0.90 x + 0.20 (90) = 0.80 (x + 90)

0.90 x + 18 = 0.80 x + 72

0.10 x = 54

x = 540

You need 540 gallons of 90% antifreeze solution.