19
23
22
21
Answer is Wrong!
Answer is Right!
The missing number is $\boxed{21}$.
The numbers in the sequence are all the numbers from 1 to 24, except for the numbers that are multiples of 3. This is because the first number in the sequence is 1, which is not a multiple of 3, and the second number in the sequence is 2, which is also not a multiple of 3. Therefore, the missing number must be a multiple of 3 that is not in the sequence. The only multiple of 3 that is not in the sequence is 21.