Hi i'm really stuck on this question. I tried to look up on youtube etc, but could'nt find anything.

Heres the question:

If 30 or more people are going on a train trip, the cost for "n" people is "$d"

d calculated using the formula "d= 60(n/4-5)

How much will the train trip cost for 48 people?

Please note I don't need the answer for this even a link to video, on how to do this would be fine. BUT I wouldn't mind a answer as well.(Nerd)