A1. At a bazaar, you can win a prize by guessing the exact number of ping pong balls in a glass jar. Arie guesses there are 90, Bea guesses there are 97, Cor guesses there are 99, and Dirk guesses there are 101. None of the four win the prize. It turns out that one of the four is off by 7, one is off by 4, and one is off by 3.

How many ping pong balls are in the jar?
A1. X is 3 away and Y is 4 away. No consecutive numbers are guessed, so X and Y are around the correct answer with a difference of 4+3=7. This difference only occurs with 90 and 97. Therefore, the correct number is 93 or 94. Someone was 7 away, and that can only be Dirk with 101. The exact number is thus 94.