(a) Let s represent the number of fruit pies sold by Mr. Sanchez's class.
Let k represent the number of bottles of juice sold by Mr Kelly's class.
s + k = 79 . . . . . . together, the classes sold 79 items
1.65s + 1.36k = 118.17 . . . . . revenue from the sales was 118.17
The system can be solved by any of several means. You can substitute for k, for example.
1.65s + 1.36(79 - s) = 118.17
0.29s = 10.73 . . . . . . subtract 1.36×79
s = 10.73/0.29 = 37
Then k = 79 - 37 = 42, and the solution is (s, k) = (37, 42).
(b) Mr Sanchez's class earned $1.65·37 = $61.05.
Mr Kelly's class earned $1.36·42 = $57.12.
Mr Sanchez's class earned more money.
(c) Mr Sanchez's class earned $61.05 - 57.12 = $3.93 more.