At gas, regular gas is $1.29 per gallon and ultra gas is $1.80 per gallon. if gas sold $69.99 worth of regular, how many gallons of ultra must be sold to have an average selling price of $1.603 per gallon.

Let g represent the number of gallons of ultra.
.. 1.603 = (1.80g +69.99)/(g +69.99/1.29)
.. 1.603(g +69.99/1.29) = 1.80g +69.99
.. 1.603(1.29g +69.99) = 1.29(1.80g +69.99)
.. 69.99(1.603 -1.29) = 1.29g(1.80 -1.603)
.. g = 69.99*(1.603 -1.29)/(1.29*(1.80 -1.603))
.. g ≈ 86.203

Edhelper Gas must sell 86.203 gallons of ultra to make the average selling price 1.603.