[Hmm. The man couldn't have known the boy's true weight, so there must be a trick to it. A drawing? No... Maybe if he had the boy stand on the paper? No, because that wouldn't be 'written'. The carnival man had to have made this an indisputable victory. Not in numbers, but in words, perhaps? He wrote the boy's exact weight...
[written] 1/2
Aha!]