I have problem here with just slight changes in the given values, this will help you to solve the problem.
Distance = Efficiency * Gallons
First car:
Distance = x
Efficiency = 20
Gallons = x/20
Second car:
Distance = 1300 - x
Efficiency = 15
Gallons = (1300 - x)/15
x/20 + (1300 - x)/15 = 75
3x/60 + 4(1300 - x)/60 = 75
3x/60 + (5200 - 4x)/60 = 75
(3x + 5200 - 4x)/60 = 75
(5200 - x)/60 = 75
5200 - x = 4500
x = 700
First car:
Gallons = x/20 = 700/20 = 35
Second car:
Gallons = (1300 - x)/15 = (1300 - 700)/15 = 600/15 = 40