I'm assuming that he rented the batting cage just once in those two months because the problem does not say anything about using the batting cage more than once. So with this assumption, the change in Casey's savings after 2 months is less 108 dollars. In order to get this, first I multiplied 6 dollars to 9 hours to get how much he spent in a batting cage in the first month. The answer to that is 54 dollars. Since he did that for 2 months, then 54 times 2. This will result in 108 dollars.
