Let x represent the number of half-gallons purchased by the store. Then x-13 is the number sold, and 2.80x is the store's cost. The revenue is 4.06(x-13) and the difference between that and cost is the profit.
... 4.06(x -13) -2.80x = 63.14
... 1.26x - 52.78 = 63.14 . . . . eliminate parentheses, collect terms
... 1.26x = 115.92 . . . . . . . . . . add 52.78
... x = 115.92/1.26 = 92 . . . . . divide by the coefficient of x
The store purchased 92 half-gallons of milk.
