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Using the information that:

Entry for an:

Adult - £3.50
Pensioner - £1.00
Child - £0.85

I have figured out that there must be 50 children, 3 adults and 47 pensioners in the cinema.

Overall:

The children paid £42.50
The adults paid £10.50
The pensioners paid £47.00

The total amount of people was 100 and the total amount of money was £100, therefore the answer was right.