"The old cars are small and no good' seems like an equally fine answer to me, in spite of it being tossed out. I'll run it by the DuoLingo team and see what they say. "Not good = no good". No?
I'm inclined to say yes, although there does seem to be a slight difference. If you say something is no good, you're usually not simply saying it's not good but that it's not suitable or fitting for some specific purpose.
Well, perhaps you're right. "No good" on second read, does sound a bit more harsh then "not good". "No good" may be heard as, "good for nothing!". Cheers for the nuance! ~ Max