Can someone me solve and explain?

question:

suppose a different tournament that eddie organizes has 120 players and the entry fee is $40. each time he increases the fee by $5, he loses10 players. determine what the entry fee should be to maximize the value of the pool.

The Answer would be 100
My reason:
First you see how many times you can take away 10 from 120 to get 0 and the answer is 12. Then you do 12 for the amount of times to increase your entry fee times 5 for the amount of increase and that is 60, lastly you do 60 + 40 which was our beginning number and you get 100.

It would be around 260 feet