Answer :

Here's a story: I had 5 dollars in my piggy bank. My mother gave me 7 more dollars for cleaning my room. I now have 12 dollars. (:
One possible story is:

I had 5 apples.  My sister gave me 7, and now I have 12.

Explanation:

We are writing a story for 5+7=12.  This means we need to start with 5 objects and somehow gain 7 more.  In our story, we start with 5 apples and we get 7 more.

The last part is that our total must be 12.  In our story, we end with 12 apples.