John has started at a job he works every day 10 days ago.  So far, he's made $250.  How many more days must John work before he makes twice this amount, assuming he makes the same amount every day?
John has made $250 in 10 days, so this means he makes $250/10=
<gadget id="calculator">250/10</gadget>
<output>25</output>
25 dollars a day.
Since John wants to make double his current amount, this means he wants to make 250*2=
<gadget id="calculator">250*2</gadget>
<output>500</output>
500 dollars.
This means John needs to work 500/25= 
<gadget id="calculator">500/25</gadget>
<output>20</output>
20 days in total to make 500.
As John has already worked 10 days, this means he needs to work 20-10=
<gadget id="calculator">20-10</gadget>
<output>10</output>
10 more days to make his goal.

<result>10</result>