The reason it is "gli" normally is because "uomini" begins with a vowel. But "uomini" is still masculine plural, so when you put a word like "cinque" (which does not begin with a vowel) in front, you use "i" instead.
So you would say "gli uomini" but "i cinque uomini", "gli elefanti" but "i cinque elefanti".
I hope that helps.
you can say: cosa fanno gli uomini? but you cannot say: cosa fanno gli 5 uomini? actually i dont know which is the rule, but i think it is about the fact that there is a vocal. EX:
gli uccelli, i pappagalli, i pesci, i cani, gli uomini, i letti, gli infermieri, gli autisti, gli esperimenti, gli occhi, i cinesi, gli africani.
as you can se with vocals we use gli, with consonants we use i. yes i think this is the rule :)
Although you were translating literally in the same order as the Italian sentence, in English '--read books' (in the order of verb + object) means it is the books that 'read' applies to, as the object being read. In speech you might say, "What do they read, (with a comma if in writing, then maybe a pause in speech as you clarify who you're referring to) the five men?" Moreover, since the Italian sentence did not include a 'they', Duo's translation 'What do the five men read?' is probably the closest translation. Hope this helps.