In following number series, two terms have been put within brackets. Choose the appropriate option - Find out the missing number in the following sequence :1, 3, 3, 6, 7, 9, ?, 12, 21.
Answer: D. 13
Clearly, the given sequence is a combination of two series : I. 1, 3, 7, ?, 21 and II. 3, 6, 9, 12 The pattern followed in I is + 2, + 4, ...; and the pattern followed in IIis + 3. Thus, missing number = 7 + 6 = 13. Hence, the answer is (d).