Since you're only interested in the "a" variable, you can eliminate r from the system by subtracting 5 times the second equation from 12 times the first.
12(5r +15a) -5(12r +11a) = 12(10.5) -5(11.45)
60r +180a -60r -55a = 126 -57.25
125a = 68.75
68.75/125 = a = 0.55 . . . . . . . the cost of each apple
