There is a ball in a box. Three kids are guessing the colour of the ball.  Val says: "The ball is white."  John says: "The ball is blue."  Elvis says: "I agree with Val."  Then, they open the box and find only one of them guessed right.  The ball is  . 
Overseas Competition->Knowledge Point->Combinatorics->Logical Reasoning->Reasoning using Hypothesis
Val\textquotesingle s point and Elvis\textquotesingle{} points are identical, so both of them guessed incorrectly.  Therefore, John guessed correctly. 