Difference between 5,6,9,15 is 1,3,6
Difference between 1,3,6 is 2,3 so the next no. in 2,3 series is 4 then the next no. in 1,3,6 series is 10 so the next no. in 5,6,9,15 is 25. Now for the cross-check
The difference between 5,6,9,15,25,40 is 1,3,6,10,15 then their difference will be 2,3,4,5 so the ans will be 25.