Answer: they have been selling tickets for 7 minutes.
Step-by-step explanation:
Let x represent the number of minutes for which they sold tickets.
The student council started the year with a balance of −$15.
By selling tickets to a student vs. faculty mud fight, the fund grew by $15 a minute. It means that in x minutes, the funds would be
15 × x = $15x
By the time the student council had a balance of $90. The expression would be
- 15 + 15x = 90
15x = 90 + 15
15x = 105
x = 105/15
x = 7