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: 18, 15, ?, 9

• The order of numbers is going down or descending.
• The difference between numbers is 18 - 15 = 3
• Since the order is descending subtract 3 from 15. The missing number may be 12.
• The missing number is 12 since it is 3 more than the last number 9.