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 - 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.