Doubt from atomic physics

Question 25 i have a doubt i think my answer is correct not theirs please can anyone clear my doubt

It is 40. You must've done some calculation mistake...

Yes I am getting 41

Tell your formula of doing this question

It should be 41, I mistakenly put Z instead of Z-1 in the formula....
My bad

So it means my answer is correct. I got it see in result it shows this question has been correctly attempted by only 4% which surely indicates that their answer key is wrong and neither will they rectify it.

Yuup... you are right.