Independent variables don't rely on anything. This will be $20. You pay $20 regardless.
Dependent variables will affect the outcome. This will be the number of friends. If f (the number of friends) ≥ 5, than you pay $3 per person.
c = cost
if f < 5
c = 20
if f ≥ 5,
c = 20 + 3f
Table
f c($)
5 35
6 38
7 41
8 44
9 47
10 50
11 53
12 56