Explanation:

Given series,
40 , 50 , 61, 73, 86, ? , 115
Difference between two number like as
50-40 = 10,
61-50 = 11,
73- 61= 12,
86 - 73 = 13 ...... as so on.
Then our difference series 10, 11, 12, 13, 14, 15
Hence, next term will be =86+14=100