"Matěj kupuje svůj patnáctý dům."

Translation: Matěj is buying his fifteenth house.

[lobachevsky.]

In this, as in many other sentences, I think that the simple present should be accepted as well.

[vor23]

The present progressive is needed here, because a person can buy his fifteenth house only once. The simple present is for a repeated or usual action.

[Kiki685123]

Why don't you accept numbers?

[endless_sleeper]

You are here to learn the language. Not numbers.

[Kiki685123]

You sound like my Latin teacher 30 years (thirty) ago. Thank you :)

[JovicaIgnj]

I think it should be ok to write numbers in English but to force letters in Czech

[whitleyadam]

I definitely agree - allow 1st, 2nd, 3rd, 10th, 15th etc.