2.3 In a closed bag lie apples. Three friends tried to lift the bag and guess how many fruits were inside. The first one said that, in his opinion, there were 16 apples, the second thought there were 19, and the third said 25. When the friends opened the bag, it turned out that one of them was off by 2, another by 4, and the third by 5. How many apples were in the bag? Find all possible answers.

#
# Answer: 21

Solution. Notice that the second and third friends give odd predictions, which means they make an error of one parity, while the first makes an error of the other parity. Therefore, the first one was off by 5 apples, and the number of apples in the bag is either 11 or 21. Checking both options, we get the answer.