The problem asks for the missing number in the sequence: 9, 18, 27, 36, __?

Let's analyze the pattern:

• From 9 to 18, the difference is $18 - 9 = 9$.
• From 18 to 27, the difference is $27 - 18 = 9$.
• From 27 to 36, the difference is $36 - 27 = 9$.

So, this is an arithmetic sequence where each term increases by 9. The next number should be:

$36 + 9 = 45$

Thus, the missing number is 45.

Would you like more details on arithmetic sequences or have any other questions?

Here are some related questions:

1. How can you identify patterns in number sequences?
2. What is the formula for the nth term of an arithmetic sequence?
3. How do arithmetic sequences differ from geometric sequences?
4. What are common types of sequences in mathematics?
5. How can arithmetic sequences be applied in real-world scenarios?

Tip: For arithmetic sequences, the difference between consecutive terms is always constant.