9
3
6
1
Answer is Right!
Answer is Wrong!
The answer is $\boxed{3}$.
The missing number is the number that completes the following pattern:
$1, 3, 6, 9, 12, 15, \boxed{18}, 21, 24, 27, 30$
This is a simple arithmetic sequence with a common difference of $3$. In other words, each term is $3$ more than the term before it. Therefore, the missing number is $3$ more than the last number in the sequence, which is $15$.