Explanation

The pattern in the series is:

×1, ×1, ×2, ×1.5, ×2, ×3...

Applying the pattern:

1 × 1 = 1

1 × 1 = 2

2 × 1.5 = 3

3 × 2 = 6

6 × 1.5 = 9

9 × 2 = 18

18 × 1.5 = 27