"What kind of trains are slow?"
Translation:Milyen vonatok lassúak?
Well ... One could have written "what kinds of trains" and "milyenek vonatok". I don't see a difference but I do get irritated by the strict corrections when it comes to there plural vs singular form. It seems arbitrary in these exercises
"What kinds of trains", yes, but not milyenek vonatok - adjectives don't take plural endings when they are used together with a noun like that.