I think there is a small error in the problem. The second sentence says Jack made 3 pieces. I think Jack actually made 3 pies. I will set up the problem as such.
Let 3 = number of pies Jack made.
y = number of pies Jill made.
Then 3 (8) + y (8) = 72
24 + 8y = 72
8y = 48
y = 6 pies