Find the odd one in the number series given below ? 17, 9, 10, 18.5, 35, 90

The given series pattern follows

17       9       10       18.5       35       90

× 0.5 + 0.5,   × 1 + 1,   × 1.5 + 1.5,   × 2 + 2,   × 2.5 + 2.5

So here 

17x0.5 + 0.5 = 9

9x1 + 1 = 10

10x1.5 + 1.5 = 16.5

16.5x2 + 2 = 35

35x2.5 + 2.5 = 90

Here the odd man in the given series is 18.5