Answer: Economic Surplus : $12
Explanation: Economic surplus is the difference between benefit and cost.
In this case, since he is working as a math tutor, he will get $45, there is no other cost. But if he goes for a movie, he will have to spend $12 on movie, which is the cost he needs to bear. The economic surplus is $12 which is not spent on the movie and worked as a math tutor.