Answer :

answer is 20n
you add like terms (same variable)

hope this helped, if im wrong....... sorry
AL2006
Well, whenever you have  5  of something plus  15  of the same thing,
you always wind up with  20  of them.  They could be cows, rocks, cars,
or pencils.  In this particular case, the things just happen to be  'n's .

So an equivalent expression for " 5n + 15n " could be  " 20n ".