The missing number is 14.
The numbers in the sequence are all multiples of 7, except for 11. 11 is the only number in the sequence that is not a multiple of 7. Therefore, the missing number must be a multiple of 7 that is not 11. The only multiple of 7 that is not 11 and is also in the range of the other numbers in the sequence is 14. Therefore, the missing number is 14.
Here is a table of the numbers in the sequence:
Number | Multiple of 7?
——- | ——–
1 | Yes
2 | Yes
3 | Yes
4 | Yes
5 | Yes
6 | Yes
7 | Yes
8 | Yes
9 | Yes
10 | Yes
11 | No
12 | Yes
13 | Yes
14 | Yes
15 | Yes
16 | Yes
17 | Yes
18 | Yes
19 | Yes
20 | Yes
As you can see, all of the numbers in the sequence are multiples of 7, except for 11. Therefore, the missing number must be a multiple of 7 that is not 11. The only multiple of 7 that is not 11 and is also in the range of the other numbers in the sequence is 14. Therefore, the missing number is 14.