Explanation:

As per the given question,
7 , 17 , 41 , 85 , ? , 257
Take difference between given number
17-7 = 10,
41 - 17 = 24,
85 - 41= 44
Hence, series will be as per the given,
10, 24, 44 .....
Again taking the difference of the given term of the series with the next term.
24 -10 =14,
44 - 24 = 20 .... as so on.
Hence the required series will be 14, 20, 26, 32 ......
Then 44 + 26 = 70
so our series
7, 17 , 41, 85, 155 , 257
= 155