How to find a missing number in a sequence

- Determine if the order of numbers is ascending (getting larger in value) or descending (becoming smaller
in value).
- Find the difference between numbers that are next to each other.
- Use the difference between numbers to find the missing number.

Example: Find the missing number: 21, 17, ?, 9

- The order of numbers is going down or descending.
- The difference between numbers is 21 - 17 = 4
- Since the order is descending subtract 4 from 17. The missing number may be 13.
- The missing number is 13 since it is 4 more than the last number 9.