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SO because a kid is less than a pound we must use quite a lot of them and 1 adult to get the equivalent amount of people. I found that you first should make a table like this
Table for 0.85 x multiples of 10
0.85 x 10 8.50
0.85 x 20 17
0.85 x 30 25.50
0.85 x 40 34
0.85 x 50 42.50
0.85 x 60 51
0.85 x 70 59.50
0.85 x 80 68
Eventually i discovered that if you use 50 x 0.85 it is equal 42.50 and then if you add 3 grownups you get 3 x 3.50 which is equal to 10.50 then if you add it to the sum of the kids you get 50 kids + 3 adults = 53 pounds so from then on you just add 47 pensioners giving you the answer
100 pounds = 50 kids, 3 adults and 47 pensioners.
I believe that this question may have other multiple answers.
there is the solution.