Look at this series: 7, 10, 8, 11, 9, 12, ... What number should come next?

This is a simple division series; each number is one-half of the previous number.

In other terms to say, the number is divided by 2 successively to get the next result.

4/2 = 2
2/2 = 1
1/2 = 1/2
(1/2)/2 = 1/4
(1/4)/2 = 1/8 and so on.

This is an alternating number subtraction series. First, 2 is subtracted, then 4, then 2, and so on.

In this simple alternating subtraction and addition series; 1 is subtracted, then 2 is added, and so on.

In this series, each number is repeated, then 13 is subtracted to arrive at the next number.

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.

This is a simple subtraction series. Each number is 6 less than the previous number.