"Their son bought four hundred and fifty-one bears."

Translation: Jejich syn koupil čtyři sta padesát jedna medvědů.

[iad58g]

Why jedna?

[Renardo_11]

There is a note that numbers greater than 20 (21, 22, 31, 32, …) always use jedna and dva, never jeden / jedno or dvě. Sorry, I cannot find the note right now.

[Ramitroj]

In standard Czech, however, it is correct to say: "Jejich syn koupil čtyři sta padesát jednoho medvěda"

[VladaFu]

It is a less-common way but it is accepted here.