"Mister, why are cars fast?"
Translation:아저씨, 자동차는 왜 빠릅니까?
I commented this when the app was first released. I suppose they have retroactively added that translation. Good to know I was correct though :)
차 can be an abbreviation of 자동차 and mean "car", but 들 is not used in general statements. So if you use "차들이" that would be "Why are the cars fast?" (as in some specific cars you are pointing to), rather than asking why cars in general are fast (which is what this exercise is looking for).
How does 빨리 become 빠릅니까? 이 in the first phrase and 으 in the second phrase are different vowels surely...