3 4 6 9 13 18 24
If you watch carefully, between the first two numbers 1 was added.
Then 2 was added to 4 to have 6.
3 added to 6 to give 9.
4 added to 9 to result to 13.
5 added to 13 to give 18.
6 added to 18 to give 24.
Therefore 7 would be added to 24 to give: 24 + 7 = 31.
So it is 31. Cheers.
