Hi fabxx,

Im not really sure if i interpreted the question right but this is one way you could solve it,

represent weight of adult by variable A

represent the average weight of a child as variable C

We are told that A = 4C

The total weight in the elevator (10 children and 1 adult) is therefore

= 14 C

The adults fraction of the total weight of the lift is then

4C/14C, variable Cs cancel and your left with the adults weight as a

fraction of 2/7 of the entire lift weight

hope that helps

Cheers