251, (12), 107, 381, (?) , 125

The given series is in the below pattern,we can see that
Square root of ( First term - Third term ) = Second term
⇒ Square root of ( 251 – 107 ) = Square root of 144 = 12 = Second term
Square root of ( Fourth term - sixth term ) = Fifth term
⇒ 381 – 125 = Square root of 256 = 16 = Fifth term
∴ Fifth term ( ? ) = 16
Hence, 16 is the missing number in the given question.