In each series, look for the degree and direction of change between the numbers. In other words, do the numbers increase or decrease, and by how much
Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next? Choose the correct choice from the following:
A) 14
B) 15
C) 16
D) 17
E) 18
15   15   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.
<result>B</result>
In each series, look for the degree and direction of change between the numbers. In other words, do the numbers increase or decrease, and by how much
Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next?