"That will do."

The woman left the room. Daubrecq returned to his letter-writing. Then,
stretching out his arm, he made some marks on a white writing-tablet, at
the end of his desk, and rested it against the desk, as though he wished
to keep it in sight. The marks were figures; and Lupin was able to read
the following subtraction-sum:

"9 - 8 = 1"

And Daubrecq, speaking between his teeth, thoughtfully uttered the
syllables:

"Eight from nine leaves one... There's not a doubt about that," he added,
aloud. He wrote one more letter, a very short one, and addressed the
envelope with an inscription which Lupin was able to decipher when the
letter was placed beside the writing-tablet:

"To Monsieur Prasville,
Secretary-general of the Prefecture of Police."

Then he rang the bell again:

"Clemence," he said, to the portress, "did you go to school as a child?"

"Yes, sir, of course I did."

"And were you taught arithmetic?"