When we talk about two things put an article before each countable noun.
Compare these sentences:
- I have a black and white dog.
(one article/a – one dog that is black and white)
I have a black and a white dog.
(two articles/a – two dogs, one is black, and one is white) - A bag of apples and oranges.
(one article/a – one bag with apples and oranges in it)
A bag of apples and a bag of oranges.
(two articles/a – two bags/one bag of apples and one bag of oranges)
