If Mark weighs 150 pounds and Susan weighs 20 pounds less than Mark.  And their friend Bob weighs twice as much as Susan.  What is the average weight of the 3 friends?
Susan weighs 150 pounds - 20 pounds = 
<gadget id="calculator">150-20</gadget>
<output>130</output>
130 pounds.
Bob weighs 2 * 130 pounds = 
<gadget id="calculator">2*130</gadget>
<output>260</output>
260 pounds.
The friends total weight is 150 + 130 + 260 pounds = 
<gadget id="calculator">150+130+260</gadget>
<output>540</output>
540 pounds.
The friends' average weight is 540 pounds / 3 = 
<gadget id="calculator">540/3</gadget>
<output>180</output>
180 pounds.

<result>180</result>