Which of the following numbers will replace the question mark (?) in the given series?
73, 57, 43, 31, ?, 13

Let's check the differences between consecutive terms in the series:
73−57=16
57−43=14
43−31=12
The differences form a decreasing sequence of consecutive even numbers: 16,14,12,…
Following this logic, the next difference must be 10:

31−10=21
Let's check the next term with a difference of 8:

21−8=13
This perfectly matches the final term of the series. Therefore, the missing number is 21.