When used (combined) with the indefinite article, the common Portuguese prepositions de and em are contracted as:
 

Preposition	Indefinite Article
	Singular = "a"		Plural = "some"
	um (m)	uma(f)	uns(m)	umas (f)
+ de (of/from) =	dum	duma	duns	dumas
+ em (in/on) =	num	numa	nuns	numas