I think this is technically more correct:

365.25 / 7 = 52.178571429
52.178571429 * 420,000,000 = 21,915,000,000
(2,100,000,000 / 21,915,000,000) * 100 = 9.582477755

Which means it’s 9.58% rounded down to 2 decimals.

(This is using the common definition of a year and not a “tropical year”)