Answer:
You can try to do a direct rule of three in this exercise.
We can considerr that the relation is proportional and we know that in this case it's dorect, as the more attendees there come, the more Estelle will have to pay, so we can write this and get x:
5 attendees - 15 dollars
26 attendees - x dollars
x = 26*15 / 5 = 78 dollars
So you know how much Estelle would have to pay
