Logic & Reasoning

Look at this series: 21, 9, 21, 11, 21, 13,… What number should come next?

a. 14
b. 15
c. 21
d. 23
  1. c. In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.

Test ID: 541
Source: 501 challenging logic & reasoning problems