Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next?

Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should

1.	Look at this series: 21, 9, 21, 11, 21, 13, 21, ... What number should come next?
Answer» In this alternating repetition series, the random number 21 is interpolated every other number into an otherwise simple addition series that increases by 2, beginning with the number 9.