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: 1, 5, 7, 3, __, 1 ...Which number is missing?  Pick one
A) 2
B) 4
C) 12
D) 0
E) 13
E
13
This is an alternating addition and subtraction series. In the first pattern, 6 is added to each number to arrive at the next. In the second, 2 is subtracted from each number to arrive at the next.
<result>E</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: 1, 5, 7, 3, __, 1 ...Which number is missing?