The original price of a set of science fiction books is 90 yuan, with a 20% discount during the New Year's Day. Teacher Li bought three sets in total. How much is each set cheaper on average?
一套科幻书原价90元，元旦期间八折优惠，李老师一共买了三套，平均每套便宜了多少钱？
<gadget id="calculator">80 / 100</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">90 * (1/5)</gadget>
<output>18</output>

<result>18</result>