Hey!

Given mental Aptitude question forms a series in which next consecutive term comes by adding a natural number starting from 3,that is,

1+ 3 =4

4+ 4 =8

8+ 5 =13

13+ 6 =19

19+ 7 =26

And so on....

So the series becomes 1,4,8,13,19,26,....

And next value of the question is 19 and 26.

Hope it helped to clear your delimma.