39460

I don't the 5 adults, 20 pensioners and 75 children helps so I am going to use my own method.

List the 2 equations:
X=adults y= pensioners z= children

1. 3.5x+y+0.85z=100(money)
2. x+y+z=100(people)

So the first equation minus the second equation and you get:
2.5x-0.15z=0
2.5x=0.15z

x 0.15
- = ----
z 2.5

Times 0.15 over 2.5 by 100 because you don't want decimals. Simplify it to 3 over 50.
So this means when there is 3 adults, there are 50 children.

x=3 z=50, together they get 53 so there are 47 pensioners.

To check if I'm correct:
Adults: 3x3.5=10.5
Children: 50x0.85=42.5
10.5+42.5=53
53+47=100

It's correct! Now there are 100 people and it exactly costs £100.