We can make an equation to figure out the answer. This is a quite common equation with a flat fee and a rate above that. The equation will be the same as the line slope formula: y = mx + b.
In this case, we can say the equation is
Total = (price per gallon)(# of gallons) + flat fee
so
40.50 = .0075g + 13.50 (where g is # of gallons)
then
27 = .0075g
then
27/.0075 = g
cancel 3 from top and bottom:
9/.0025 = g
Multiply top and bottom by 4 to make the decimal easy:
36/.01
multiply top and bottom by 100:
3,600 = g
So Judy used 3,600 gallons.
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