What should come in place of '?' in the given series? 325, 306, 289, 274, 261, ?

Let's find the difference between consecutive numbers in the series:

$325 - 306 = 19$

$306 - 289 = 17$

$289 - 274 = 15$

$274 - 261 = 13$

The pattern of differences is a decreasing sequence of odd numbers: 19, 17, 15, 13. The next difference in this sequence should be $13 - 2 = 11$.

Therefore, the next number in the series is $261 - 11 = 250$.