"Thirty one cars"
Translation:Тридцять одна машина
what's the problem with ''авто' ? (instead of 'машина') doesn't авто mean ''auto / car'' ?
Yes, it does. 'авто' is a short form of 'автомобіль'. Besides, there is 'автівка', a popular colloquial word for 'car' that is common in everyday speech and modern Ukrainian literature. All those variants should be accepted.