Help me build a puzzle

Hi,
I need to build a puzzle that in order to find it’s answer we need to solve this equation :

Thanks
Answer #1

Answer #2
@, thanks for your help, but this is not what I meant (maybe I wasn’t clear). I just need it to be a puzzle not a math one
Answer #3
No, not very clear…
You want a puzzle about equation solving that doesn’t involve maths?
Answer #4
For example, I have a group of x people and one of them is my friend, and I was told that one of them will be my dorm mate. what is the probability that my friend will be chosen, if the process is being done randomly?? 1/x.
Of course the puzzle can be anything, not only combinatorics.