$20 means Margie can buy 5 gallons . . . (i.e. $20 / $4 per gallon = 5 gallons)
we know she can go 32 miles on every gallon, so we just multiply by the 5 gallons she just bought with her $20, and find out she can make it 160 miles (i.e. 32×5 = 160)
answer is 160 miles
