It is 4.5 yuan per kilogram of oranges. Mom bought 18 kilograms of apples and paid 100 yuan. How much should I get back?
每千克橘子4.5元，妈妈买了18千克苹果，付出了100元，应找回多少钱？
<gadget id="calculator">4.5 * 18</gadget>
<output>81</output>

<gadget id="calculator">100 - 81</gadget>
<output>19</output>

<result>19</result>