Tommy wants to earn enough money to buy 8 new books. Each book costs $5. If Tommy already has $13, how much does he need to save up?
The books will cost $40 because 8 x 5 = 
<gadget id="calculator">8*5</gadget>
<output>40</output>
40.
He needs to save up $27 because 40 - 13 = 
<gadget id="calculator">40-13</gadget>
<output>27</output>
27.

<result>27</result>