To find the next number in the sequence 21, 13, 26, 31, let's look at the differences between the consecutive numbers:
- From 21 to 13, the change is -8 (21 - 13 = 8).
- From 13 to 26, the change is +13 (26 - 13 = 13).
- From 26 to 31, the change is +5 (31 - 26 = 5).
Now let's look at the pattern of these changes:
- The differences are: -8, +13, +5.
If we analyze more closely:
- The first difference (-8) could suggest a reduction.
- The second difference (+13) suggested an increase.
- The third difference (+5) is less than +13.
There seems to be no clear arithmetic pattern or simple sequence, but one way to project forward might be observing the last difference (+5) suggests a smaller increment.
If we continue with a similar pattern and choose to increment by another value slightly less than the +5, we might predict the next difference as decreasing by about 2 or so (since going from -8 to +13 increased quite a bit).
A guess could be to add another +5 to +3 or so: 31 + 3 = 34.
Ultimately, without a clear rule laid out in the pattern, the next reasonable guess in the sequence could be 34 based on continuation of adding lower positive integers/mild oscillating pattern.
Therefore, the next number could be 34. However, without more information on the logic or rules behind the sequence, this remains a speculative answer.