She can keep up!

"Five and three-quarters," she says promptly, "yes we should, and of course I do. One moment."

Matilda digs around in her backpack, producing with remarkable alacrity a notebook with a mechanical pencil shoved through the ring binding. The blank page to which she opens it is opposite a scribbled derivation of Pascal's Triangle in base thirty-six, with marginal notes on combinatorics.

"There you are."