Shows what exactly. An average 11 year old can do the arithmetic. Or the joke answer the lift is free.

So what have you found out.
Your candidate can do basic arithmetic
Or they read the 25 page paperback, questions you might be asked in an interview.

So what exactly have you said with such a question.

I have only basic arithmetic skills
I find even the simplest exercise in logic so stressful it brings out the best in me...
I have a childish sense of humour
I don't know what I'm doing but I found this book in a waiting room, it's great, better still the writing's really big, there are only few pages, so I read it in the week before I had to start interview.
And most of all
I can't be arsed, I'm just going through the motions

The real problem isn't you being stupid, or us thinking you are stupid, it's you thinking your clever because you are on the boss side of the table.