I really need help....

Could anyone explain how to do the problem because my math teacher didn't even explain how to do this kind of problem at school.

The Computer Club is going on a field trip to a computer museum. The trip costs $240, to be paid for equally by each club member. The day before the trip, however, four students decide not to go. This increases the cost of the trip by $2 for each remaining student. How many students are going to the computer museum?