2.2 In a closed bag lie apples. Three friends tried to lift the bag and guess how many fruits were inside. The first said that, in his opinion, there were 19 apples, the second thought there were 22, and the third said 23. When the friends opened the bag, it turned out that one of them was off by 1, another by 2, and the third by 3. How many apples were in the bag? Find all possible answers.
Answer: 20

Solution. Note that the first and third friends give odd predictions, which means they make errors of one parity, while the second makes errors of the other parity. Therefore, the second friend was off by 2 apples, so there are either 20 or 24 apples in the bag. Checking both options, we get the answer.