What is the missing number in the following sequence: 1, 3, 6, 11, 18... ?

The missing number in the sequence is 27.

The pattern in the sequence is adding consecutive odd numbers to the previous number.

1 + 2 = 3
3 + 3 = 6
6 + 5 = 11
11 + 7 = 18
18 + 9 = 27

Therefore, the missing number is 27.

29