What should come in place of question mark ‘?’ in the following number series?

20, 9, 8, 11, ?

- 22
- 10
- 21
- 14
- 9

Option 3 : 21

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The pattern of the given problem is:

9 = 20 × 0.5 - 1

8 = 9 × 1 - 1

11 = 8 × 1.5 - 1

21 = 11 × 2 - 1 = ?

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