That's ambiguos because "par" also means 2 separated pieces.
OK. I don't see this as particularly important but there are two contributing factors within this discussion: 1) the fact that, here in the UK, we never refer to trousers as pants. Pants are underwear. It is a peculiarity that both come in "pairs" although everyone knows they are both single item. You can't buy half a pair of pants or trousers! 2) This is a nuance of UK English but, generally, if I "buy cheap trousers" then it implies that the item is inferior in some way. If I "buy a pair of trousers cheap" then it implies that I paid less than the normal price for the item. Either way, the English is good - there is just an implied difference of meaning.
I don't think it makes grammatical sense in any English speaking country to use "cheap" as an adverb. You could use "cheaply" but I'm not sure if that's a real word. The phrase "on the cheap" is the closest accepted (though informal and maybe only used in America) form of "cheap" as an adverb.
The Oxford Dictionary says "cheap" can be an adverb (only the Learners' Dictionary is available online without a subscription: https://en.oxforddictionaries.com/definition/cheap ).
As previously said, I have no authority or expertise outside of the south of England and no idea what is correct or what is normally said in the 'States'. However, I think we need to remember what this forum is - and what it is not. Yes of course "cheap" can be an adverb but does it matter? My previous comment was about actual usage of the language and was meant to help those people from outside my area to understand how the language is used here. The fact is that the vast majority of people here would not know what an adverb is - and they couldn't care less! That does not stop them using (or abusing, if you like) the English language. We should be striving to communicate with others and should not get side-tracked by whether a "rule" has been stretched or broken. I have tried not to be too academic on this forum - hence "this be me last word" on this topic!
Daniel, I'll copy my post from above here, since it was after yours: "In a DL lesson long ago and far away, I believe it was established that, in the US, 'three pair' (as my mother said/taught) is quite old-fashioned and 'three pairs' is the modern terminology. Not saying which I prefer, only what others have said . . . ." So, I do not think "three pair" is actually incorrect.
Amy91..., why not? Both THREE of anything and Pants/trousers are plural, and take plural adjective endings when something defines them.
It seems reasonable to go along with the multi-millions of others speaking both Spanish and English, both sides of the Atlantic.
Or, we could see if they'll create a special rule just for you! ;-D
(I hope you don't get offended by a little teasing, it is meant to lighten up this discussion so we don't argue about it.)
I said "I bought three pairs of pants cheap." If I replace 3 pairs of pants with 3 ink pens and exchange it in the original sentence. Yo compré tres bolígrafos baratos. Do you say: I bought three cheap ink pens. Or do you say: I bought three ink pens cheap. The difference is the quality of the ink pens vs the cost. I think the latter one is how American English would be spoken.
Er, as a native speaker of American English, I'd like to point out that "I bought three cheap ink pens" has a somewhat different meaning than "I bought three ink pens cheap." In any case, I, personally would be much more likely to say "I bought three cheap pens" than either of your suggested variants. I can't remember when the last time I heard someone say "ink pen" was.
I agree with both. I have heard "Three pair of pants" and used it here. It may well also be regional or something falling from usage.
South Jersey US English with a significant smattering of BBC World Service in my formative years.
Reported, but admittedly we may be a minority opinion.
In American English, which is what Duolingo uses, "pants" is perfectly acceptable. If the system failed to accept "trousers," report your sentence, using the button on the lower left, after you answer the question. If the course contributors agree that your translation is appropriate, they will (eventually) add it to the list of alternative translations.