The next logical number in the series comprised of 16, 26, 21, 31, __ is 26. The difference between 16 and 26 is 10, the difference between 26 and 21 is -5, and the difference between 21 and 31 is again 10.
Logically, the next number differs from 31 by -5, so the answer is 26. A number sequence is a set of limited numbers with a relationship of some sort to which one must come up with the missing number. Number sequences often appear as problems on standardized tests. Arithmetic and geometric sequences are most frequently seen, but many sequences fall into neither category.