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.
In this alternating repetition series, a random number, 33, is interpolated every third number into a simple addition series, in which each number increases 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.
EXAMIANS