What is the 10th number in the following sequence?

3, 9, 15, 21........

The given sequence follows the pattern of adding 6 to each number to get the next number.

3 + 6 = 9
9 + 6 = 15
15 + 6 = 21

To find the 10th number, we continue adding 6 to each number until we reach the desired position.

21 + 6 = 27
27 + 6 = 33
33 + 6 = 39
39 + 6 = 45
45 + 6 = 51

Therefore, the 10th number in the sequence is 51.