There is a ball in a box and three kids are guessing what colour it is.  \textbf{Val says: "The ball is white."}  \textbf{John says: "The ball is blue."}  \textbf{Elvis says: "I agree with Val."}  They open the box and find only one of them guessed right.  What colour is the ball? 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
We can spot that Val\textquotesingle s guess and Elvis\textquotesingle{} guess are the same, so both of them must be wrong. Therefore, John guessed it right. 