"The shop sells cheap clothes."
Translation:Butikken selger billige klær.
There might be some more scientific answer, but mine is that Norwegian is notorious about compounding. You can make a new one if you like any time. Two nouns, a noun and a adjective, a noun and a verb, two verbs, I guess almost anything goes. Lots of quite common compounds won't even be in the dictionary because they are self-explanatory (to the natives).
The compound ananasbiter means "pieces of pineapple", if you write ananas biter then it says "pineapple bites". This compound is actually two nouns, but "en bit" in indecisive plural (biter) is the same as present form of the verb "å bite" (biter). Of course everyone understand that a pineapple can't bite anyone, but for the native reader splitting up compounds disrupts the flow of the language because you get that quick verb/noun connection that make your brain stumble a bit. You can also string words together with the hyphen, ananas-biter, that is virtually like a compounded word but a bit immature looking.
For someone learning the language reading tabloids can be a pain as they keep making up new compounds to make stuff rhyme in their headlines. Much like I see British papers fool around with words as well;-)
I answered "butiken selger billig klære" which was rightly flagged as incorrect, though my vocabulary and training got me very close. I find it confusing to know when to use which version of any given word or idea. How does one know when to use the numerous versions of each adjective (billig instead of billige, for example) or noun?
I believe "billig" is singular and "billige" is the plural of cheap. Since the sentence is about more than one article of clothing, we are to use the plural "billige." If you have not completed the colors exercise yet, there are some similarities there as well that may help with understanding when to use which version.