24
If you list multiples of both 3 and 4:
3, 6, 9, 12, 15, 18, 21, 24, 27
4, 8, 12, 16, 20, 24, 28
If you add another 3 and 4 to 28 or 27 it would result in being greater than 30. The next number that’s a multiple of both that’s less than 30 but greater than 20 is 24
