If it has 2 and 3 in its factorization then it will be a multiple of 6.
24 is the only number between 20 and 29 that is a multiple of 6.
The long way would be to:
List the multiples of 2:
22, 24, 26, 28
List the multiples of 3:
21, 24, 27
The one that is both is 24.