Learn Turkish: Bu yetmiş araba ucuz mu? - Are these seventy cars cheap?

Yes! While some tens like 20 (yirmi) and 50 (elli) do not look like their base numbers, 70 (yetmiş) is clearly related to 7 (yedi). The d simply hardens into a t. You will see similar patterns with 60 (altmış from altı) and 80 (seksen from sekiz).

Why is it araba instead of arabalar if there are seventy cars?

In Turkish, when you use a specific number before a noun to say how many there are, the noun must always stay in its singular form. So it is always yetmiş araba (literally "seventy car"), never yetmiş arabalar.

If the English translation says "these", why does the Turkish use bu (this) instead of bunlar (these)?

When a pointing word like bu (this) is used as an adjective directly in front of a noun phrase, it does not take a plural suffix, even if the meaning is plural. You only use bunlar when it stands alone as a pronoun, like in the sentence "These are cheap" (Bunlar ucuz).

Why is the question particle mu here instead of mı?