If s is the service fee and the cost of one ticket is t then the price p in dollars is given by:
p=s+nt where n is the number of tickets. When n=1, p=$1600, so:
1600=s+t and 7902=s+5t, therefore:
7902-1600=s+5t-s-t=4t,
6302=4t, and t=6302/4=$1575.50. So s, the service fee=1600-1575.50=$24.50.
The equation is p=24.50+1575.50n. (When n=1, p=24.50+1575.50=1600; when n=5, p=24.50+7877.50=7902.)
If you don't like algebra, the difference in the two prices must be the four tickets, so one ticket costs a quarter of this. We know the price including the service fee is $1600 so the service fee must be this less the price of a ticket.