You work these problems the same way parts A and B were worked.
C) cost of gas = (cost per gallon)*(number of gallons)
.. 71.98 = 3.999*(number of gallons)
.. 71.98/3.999 = number of gallons = 18
D) miles per gallon = miles/gallons
.. miles per gallon = (15121 -15004)/(.25*18) = 117/4.5
.. miles per gallon = 26