Tom Sawyer has tricked his friends into whitewashing Aunt Polly's 100-foot fence. His first friend, Ben, whitewashed 10 feet. His second friend Billy whitewashed a fifth of the remaining fence. A third friend, Johnny, whitewashed a third of the fence that was left after that. How much of the fence still needs to be whitewashed?
It's a 100-foot fence, and Ben whitewashed 10 feet, so there are 100 - 10 = 
<gadget id="calculator">100-10</gadget>
<output>90</output>
90 feet left after Ben is done.
Billy whitewashed a fifth of that, or 90 / 5 = 
<gadget id="calculator">90/5</gadget>
<output>18</output>
18 feet.
There is now 90 - 18 = 
<gadget id="calculator">90-18</gadget>
<output>72</output>
72 feet of fence left.
Johnny whitewashes a third of that, or 72 / 3 = 
<gadget id="calculator">72/3</gadget>
<output>24</output>
24 feet.
That leaves 72 - 24 = 
<gadget id="calculator">72-24</gadget>
<output>48</output>
48 feet of fence left to be whitewashed.

<result>48</result>