Look at this series: 15, __, 27, 27, 39, 39, ... What number should fill the blank?

This is a simple subtraction series in which a random number, 93, is interpolated as every third number. In the subtraction series, 10 is subtracted from each number to arrive at the next.

This is a multiplication series; each number is 3 times the previous number.

This is an alternating addition and subtraction series. Roman numbers alternate with Arabic numbers. In the Roman numeral pattern, each number decreases by 1. In the Arabic numeral pattern, each number increases by 1.

This is a simple alternating addition and subtraction series. The first series begins with 8 and adds 3; the second begins with 43 and subtracts 2.

In this series, 5 is added to the previous number; the number 70 is inserted as every third number.

This series alternates the addition of 4 with the subtraction of 3.

In this series, the letters progress by 2, and the numbers increase by 2.

Two series alternate here, with every third number following a different pattern. In the main series, 3 is added to each number to arrive at the next. In the alternating series, 5 is subtracted from each number to arrive at the next.

This is a simple subtraction series; each number is 4 less than the previous number.

This is a simple multiplication series. Each number is 2 times greater than the previous number.