"What, the pen costs seventy euros?"
Translation:Was, der Stift kostet siebzig Euro?
My answer: "Was, der Stift kostet siebzig Euros?" Duolingo's explanation of why it's wrong: "You used the plural "Euros" here, instead of the singular "Euro"." Well, it's seventy, not one, so yes, the plural seems to be the obvious choice... Can someone explain this to me please!? Thanks!
German uses the singular for most units of measurement, e.g. zehn Kilo, drei Pfund, sieben Euro "ten kilograms, three pounds, seven euros".
The plural would only be used when you are speaking about multiples of a particular unit -- so sieben Euros would only be used if you have seven things that are each an Euro: seven one-euro coins. But if you have a €5 note and a €2 coin, you have the sum of €7, but you don't have seven things in your hand: it's sieben Euro but not sieben Euros.
Similarly, if you're advertising a diet, you might say that it lets die Pfunde purzeln "the pounds fall away", because you're imagining the pounds falling off one by one.