Question

3,4,7,7,13,13,21,22,31,34 how to calculate next number

Answer 1

The series is of combination of two series:

Series one is 3, 7, 13, 21, 31,....

Series two is 4, 7, 13, 22, 34,.....

The pattern in first series is + 4, + 6, + 8, + 10,.....

The pattern in second series is + 3, + 6, + 9, + 12,.....

So, the next term is= 31 + 12 = 43.

Answer 2

This is a two series combination problem.
Take the alternate numbers and form two new series.
1st series - 
3    7    13    21    31       ?
  +4  +6    +8   +10  +12
2nd series - 
4    7    13    22    34
  +3  +6   +9   +12

Therefore, the answer is 31+12=43
May this help you.....