Paragraph: Start off with these simple series of numbers. Number...

Paragraph: Start off with these simple series of numbers. Number series questions measure your ability to reason without words. To answer these questions, you must determine the pattern of the numbers in each series before you will be able to choose which number comes next. These questions involve only simple arithmetic. Although most number series items progress by adding or subtracting, some questions involve simple multiplication or division. 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?Question: Look at this series: $21,9,21,11,21,13, \ldots$ What number should come next?