For finding the solution, you want to sum the revenue from each class of paying customers. There were 92-40 who were over 12 years and paid $48, so the revenue from those folks is (92-40)·48. This term is found in the first and last selections only.
There were 40-x customers who were in the age range 3–12 years, so paid $36 each. The revenue from them is (40-x)·36. This term is found in the last selection only.
The appropriate choice is ...
... D.) (40 - x)·36 +(92 - 40)·48 = C
