6,13,___,17,10,21

To find the missing number in the sequence 6, 13, ___, 17, 10, 21, we need to identify the pattern or rule that governs the sequence.

Looking at the given numbers, we can observe the following:

6 + 7 = 13
13 - 6 = 7
17 - 6 = 11
10 + 11 = 21

From this observation, it appears that the sequence alternates between adding and subtracting the number 6. So, to find the missing number, we can apply the same rule.

Starting with the last number in the given sequence, which is 21, we need to subtract 6 since the pattern suggests subtraction after adding:

21 - 6 = 15

Therefore, the missing number in the sequence is 15.

To summarize, we identified the pattern in the sequence by observing the relationship between consecutive numbers. By alternately adding and subtracting 6, we found that the missing number is 15.