John drives 1000 miles a month.  He needs to get an oil change every 3000 miles.  He gets 1 free oil change a year.  If an oil change costs $50, how much does he pay a year?
He needs to get an oil change every 3000/1000 = 
<gadget id="calculator">3000/1000</gadget>
<output>3</output>
3 months.
So he gets 12/3 = 
<gadget id="calculator">12/3</gadget>
<output>4</output>
4 oil changes a year.
That means he pays for 4-1 = 
<gadget id="calculator">4-1</gadget>
<output>3</output>
3 oil changes a year.
So he pays 3*$50 = $
<gadget id="calculator">3*50</gadget>
<output>150</output>
150 a year.

<result>150</result>