3, 7, 15, 27, 63, 127, 255

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Question:
3, 7, 15, 27, 63, 127, 255

[A].

7

[B].

15

[C].

27

[D].

63

Answer: Option C

Explanation:

Go on multiplying the number by 2 and adding 1 to it to get the next number.

So, 27 is wrong.