Answer:
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.
