Filipinos often use either ng or na to connect two words, usually ang adjective and a noun.
If the word that appears before the linker ends with a vowel they use ng, if it ends with a consonant they use na like in the following two examples:
Masamang kalagayan (bad situation)
Mahirap na kalagayan (difficult situation)
So, in the first example, because masama ends with a vowel I used ng, while after mahirap that ends in a consonant, I used na.
The same applies when you are linking two numbers like:
Apat na raan