Here is my 100% Achievement Guide!
After grabbing the toilet paper for the three weirdos in a toilet cubicle together, you give them that and tog ive you your required item (God forbid what you think it is haha)
You need to solve 4 riddles.. SADLY though, a few puzzles in this game are randomized and this is the first one.
Your not timed, so dont rush, theres no pressure.
Hopefully you get most of, if not the same ones as me but theres no gurantee.
I think theres about 8 riddles they could tell you and i can't find any place where they have all the answers for the riddles.
But keep trying, and eventually it will easily come to you.
Go to 16:30 in the video to see how it is done.
Hope this helps!