Because, madam, he is basically incapable of understanding the problem.

7 seconds sound clip from the Mr. Topaze (1961) movie soundboard.

You can hear this line at 00:37:11 in the Blu-ray version of the movie.

Quote context

[...]

- Here he had another nought yesterday for mathematics.

- There's no doubt about it-the last in the class is Pitard Verniard.

- And why is my grandson bottom of the class?

- Why is he bottom?

- Because he keeps getting noughts.

- But why does he keep getting noughts?

- Yes, why?

- Because, madam, he is basically incapable of understanding the problem.

- And why is he basically incapable of understanding the problem?

- I will tell you!

- The Baroness will tell you.

- My grandson does not understand the problem because the whole thing is pre-arranged.

[...]