Given:
The equation
represents the total amount T, in dollars, Salim will pay for n tickets.
To find:
What does 12 represent in the equation?
Solution:
The vendor charges a one-time service fee for processing the purchase of the tickets.
We have, the equation

Here, 15 is the coefficient of n and 12 is constant.
If n is number of tickets then 15 must be the price of each ticket.
12 is free from variable and it is fixed. So, it represents one-time service fee for processing the purchase of the tickets.
Therefore, 12 represents that one-time service fee for processing the purchase of the tickets is $12.