Our school has always taught students to be "thrifty and filial to their parents". One day, Zhang Ming's mother gave him 20 yuan as pocket money for a week, and Zhang Ming only wanted (4/5). You know filial piety How much did Zhang Ming refund to his mother?
我校一直教育学生要"勤俭节约，理解孝顺父母"，有一天，张明的妈妈给了20元给他当作一周的零花钱，而张明只要了(4/5)，你知道孝顺的张明退了多少钱给妈妈吗？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">20 * (1/5)</gadget>
<output>4</output>

<result>4</result>