Mary was working on a research paper and already had borrowed 5 books from the library.  3 books weren't helpful so she returned those and checked out 5 more books.  3 days later, she went back to the library and returned 2 of those books and checked out 7 more books.  How many books does Mary currently have checked out of the library?
Mary started with 5 books and returned 3 so she had 5-3 = 
<gadget id="calculator">5-3</gadget>
<output>2</output>
2 books.
She had 2 books and checked out 5 more books so she had 2+5 = 
<gadget id="calculator">2+5</gadget>
<output>7</output>
7 books.
She now has 7 books but returned 2 so she has 7-2 = 
<gadget id="calculator">7-2</gadget>
<output>5</output>
5 books.
She had 5 books and checked out 7 more books so she has 5+7 = 
<gadget id="calculator">5+7</gadget>
<output>12</output>
12 books checked out.

<result>12</result>