During the "May 1st" period, all children's books in Xinhua Bookstore will be sold at a 20% discount. Xiaohua bought a set of picture books and spent 72 yuan. How much is this set of picture books cheaper than the original price?
"五一"期间，新华书店所有少儿图书打八折出售．小华买了一套图画书，花了72元．这套图画书比原价便宜了多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">72 / (4/5)</gadget>
<output>90</output>

<gadget id="calculator">90 - 72</gadget>
<output>18</output>

<result>18</result>