What should come in place of the question mark (?) in the given series?
23,44,65,86,?,128

Let's check the differences between successive terms in the series:
44−23=21

65−44=21

86−65=21

The pattern is a constant addition of +21 to the previous term.

Following this rule:

Missing number=86+21=107
Let's cross-verify with the last term:

107+21=128
This matches the given series perfectly.