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To approach this question I tried doing using trial and error using the spreadsheet which they provided. The closest I could get to £100 was £100.05, when there were 5 adults 12 pensioners and 83 children. I have come to a conclusion that the only way of getting exactly £100 is if there were either not enough or too many people. One way which you could get £100 with 100 people is when there are 100 pensioners, however it states that Alison knew that the audience was not full of pensioners. Unless 15 of the children sat on their grandparents laps it is not possible.