A hamburger is 15 yuan. If you buy 3 hamburgers and pay 100 yuan, how much should you get back?
一个汉堡包15元，买3个汉堡包，付了100元，应找回多少元．
<gadget id="calculator">15 * 3</gadget>
<output>45</output>

<gadget id="calculator">100 - 45</gadget>
<output>55</output>

<result>55</result>