Complete the series with full steps
? , 17, 33, 51, 75

Dear Student,

Sum of digits of 17 = 1 +7 =8
Now , 8 × 2 = 16   and     17 + 16 = 33

Sum of digits of 33 = 3 + 3 = 6
Now, 6 × 3 = 18    and    33 + 18 = 51 

Sum of digits of 51 = 5 + 1 = 6
Now,  6 × 4 = 24    and   51 + 24 = 75 

Similarly,  to get 17 the previous number should be 13.
As , sum of digits of 13 = 3 + 1 = 4
And 4 × 1 = 4   and    4 + 13  = 17

So, 13 is the missing  number.

Regards,