It takes 10 minutes to wash a car, 15 minutes to change oil, and 30 minutes to change a set of tires.  If mike washes 9 cars, changes the oil on 6 cars, and changes two sets of tires how many hours did he work?
He spent 10 minutes *9 cars = 
<gadget id="calculator">10*9</gadget>
<output>90</output>
90 minutes washing cars.
He spent another 15 minutes * 6 cars = 
<gadget id="calculator">15*6</gadget>
<output>90</output>
90 minutes changing oil.
He spent 2 cars * 30 minutes = 
<gadget id="calculator">2*30</gadget>
<output>60</output>
60 minutes changing tires.
He spent 90 minutes + 90 minutes + 60 minutes = 
<gadget id="calculator">90+90+60</gadget>
<output>240</output>
240 minutes working.
He spent 240 minutes / 60 minutes = 
<gadget id="calculator">240/60</gadget>
<output>4</output>
4 hours working.

<result>4</result>