Given:
32 miles on a gallon of gas
$4 per gallon.
Ratio of miles to cost of gallon.
32 : 4
Out of the $20 worth of gas, there is 5 gallons.
$20 / $4 = 5 gallons.
Proportion:
32 : 1 = x : 5
32*5 = 1x
160 = 1x
Margie can drive 160 miles on 5 gallons gas worth $20.