Which number will replace the question mark (?) in the following series?
1, 4, 3, 4, 7, 7, 13, 13, ?
it is an alternative series. where 1,3,7,13 in a series and 4,4,7,13 are also in a series.
$$1+2=3$$
$$3+4=7$$
$$7+6=13$$
$$13+8=21$$
So, next number is 21 in the above series.
option D is correct.
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