A charity group decides to do a yard sale.  10 people donate 5 boxes of stuff each.  They also have 10 boxes of stuff already.  They can fit 2 boxes worth of stuff per table.  If they already own 15 tables, how many new tables do they need?
There were 10*5=
<gadget id="calculator">10*5</gadget>
<output>50</output>
50 donated boxes.
So they have a total of 50+10=
<gadget id="calculator">50+10</gadget>
<output>60</output>
60 boxes.
So they need a total of 60/2=
<gadget id="calculator">60/2</gadget>
<output>30</output>
30 tables.
That means they need 30-15=
<gadget id="calculator">30-15</gadget>
<output>15</output>
15 tables.

<result>15</result>