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: 15, 13, ?, 9

• The order of numbers is going down or descending.
• The difference between numbers is 15 - 13 = 2
• Since the order is descending subtract 2 from 13. The missing number may be 11.
• The missing number is 11 since it is 2 more than the last number 9.