Find the missing number in the following sequence.

1,3,3,6,7,9,?,12,21

A) 11

B) 13

C) 15

D) 17

It's a combination of two series

1,3,7,?,21....... and 3,6,9,12.........

The pattern followed in first series is +2,+4,+6.....

and in second +3

so the missing no is 7 + 6 = 13

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