Which of the following numbers will replace the question mark (?) in the given series?
6, 8, 13, 23, 40, ?

6, 8, 13, 23, 40, ?

(I) Difference of two constitutive number is increasing from 2, 5, 10, 17.

8 - 6 = 2

13 - 8 = 5

23 - 13 = 10

40 - 23 = 17

(II) The differences between the differences are increasing by : 3, 5, 7.

5 - 2 = 3

10 - 5 = 5

17 - 10 = 7

So, the next difference should increase by 2:
7 + 2 = 9.

Now, we add this 9 to the last difference (17)

17 + 9 = 26

add this 26 to the last number in the series (40):

40 + 26 = 66

The next number in the series is 66.

Thus, correct option is (b)​​