Odd man out of the series 90 72 56 42 36 20

The given number series is 90 72 56 42 36 20

9 x 10 = 90

8 x 9 = 72

7 x 8 = 56

6 x 7 = 42

5 x 6 = 30 (not 36) 

4 x 5 = 20

Hence, the odd man in the given number series is 36.