Connor wanted to order some new books to read.  The website was offering free shipping over $50.00.  They also had a deal that when you buy 4 books; the first two were 25% off.  Book 1 cost 13.00 and Book 2 cost 15.00.  Book 3 & 4 were both $10.00 each.  How much more money did Connor need to spend to receive free shipping?
Book 1 was $12.50 and book 2 was $15.00 so 13.00 +15.00 = $28.00.
He was buying 4 books, so these 2 were 25% off so 28.00 * .25 = $7.00.
These two books came out to 28-7 = $
<gadget id="calculator">28-7</gadget>
<output>21</output>
21.00.
Books 3 &amp; 4 were both $10 each so 2*10 = $20.00.
When you add all 4 books together, 21+20 = $
<gadget id="calculator">21+20</gadget>
<output>41</output>
41.00.
To get free shipping he would need to spend 50-41 = $
<gadget id="calculator">50-41</gadget>
<output>9</output>
9.00 more on books.

<result>9</result>