where did j.k. rowling come up with the number 9 3/4 for the gate for the hogwarts express?

Question

and what's with the 33.1 in smallville, or the 10/6 on the mad hatter's hat...

any significance at all?

Answer 1

9 3/4 is because it is in the post between platform 9 and 10 3/4 from 9. mad hatter is the hat size.

Answer 2

Because it's a dumb trick that's been used in many, many other stories. How did she become so famous and rich "writing" (recycling) that cliched stuff, anyway?