8 11 21 15 18 21 22 , 8 11 21 15 18 21 22

A. 25 18

B. 25 21

C. 25 29

D. 22 26

Correct Answer :

B. 25 21

Explanation

This is an alternating addition series, with a random number, 21, interpolated as every third number. The addition series alternates between adding 3 and adding 4. The number 21 appears after each number arrived at by adding 3.