First we need to find the number of gallon she took in the trip
the number of gallons = 172.5/30
the number of gallons = 5.75
She took 5.75 gallon of gas
Second, find the cost per gallon
the cost = gallons cost/the number of gallons
the cost = 20.70/5.75
the cost = 3.6
The approximate cost of gas per gallon is $3.60