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.