Aptitude - Aptitude-Number Series

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This is a simple addition series with a random number, 18, interpolated as every third number. In the series, 4 is added to each number except 18, to arrive at the next number.

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In this simple subtraction series, each  number is 6 less than the previous number.

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This is an alternating repetition series, in which a random number, 61, is interpolated as every third number into an otherwise  simple subtraction series. Starting with the second number, 57, each number (except 61)
is 7 less than the previous number.

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This is a simple subtraction with repetition series. It begins with 20, which is
repeated, then 3 is subtracted, resulting in 17, which is repeated, and so on.

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This is an alternating subtraction series with repetition. There are two different patterns here. In the first, a number repeats itself; then 3 is added to that number to arrive at the next number, which also repeats. This gives the series 17, 17, 20, 20, 23, and so on. Every third number follows a second pattern, in which 3 is subtracted from each number to arrive at the next: 34, 31, 28.