39450

There will be 10 adults, 31 pensioners, and 40 children in total to make 100 pounds.

First I figured that there should be more children then pensioners, as the tickets for the children costed less, then I figure that there should be more pensioners then adults, as they cost less.
I first put 7 adults, 30 pensioners, and 40 children, this gave me a total of 92.75 pounds, I then lowered the amount of children to 40 and increased the adults to 10, there were still 30 pensioners, this gave me 99 pounds. Since I only needed one more pound I just put in one more pensioner. This gave me 10 adults, 31 pensioners, and 40 children to make a grand total of 100 pounds.