How much does your new house cost?
Three thousand US dollars.
How much US dollars does your new house cost?
Three thousands.
Are the two answers both correct?
I am not sure in which situation I should use a s and in which not.
No, not if you are giving an exact figure. The dollars are plural, the number is just a number. The only time you would make thousand plural is if you wish to give a general impression of the sort of amount you are talking about. "Was the necklace worth hundreds of dollars?" "No, thousands."
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Not all currency names have a separate plural form. Dollars do and, officially, Pounds do; although sometimes people do say "I'll give you five pound for that". The European Euro, the Chinese Yuan and the Japanese Yen, however, do not have a separate plural so the officially correct expressions are "500 Euro", "5000 Yuan" and "50000 Yen". In the case of the Asian currencies this is probably because Chinese and Japanese do not have explicit plurals for most words: in the case of the Euro I suspect that it is because the plural form is different in different European languages.– BoldBenApr 30 '19 at 8:28
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2@BoldBen I agree that you don't hear "euros" on the continent, but British people do say "euros" as well as "euro" - a bit like pound and pounds.– WS2Apr 30 '19 at 9:01
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The other unusual case is "two threes are six" or "two threes make six" meaning 3 x 2 = 6. Or referring to any thing labelled with a number, such as £20 and £10 notes: "Can I give you a twenty for two tens? May 1 '19 at 11:58
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@WS2 You do hear it, but the official definition specifically excludes a separate plural. The Irish, who primarily speak English but are in the Euro zone say "100 Euro" rather than "100 Euros".– BoldBenMay 1 '19 at 17:12