The original price of a pen is 20 yuan, how much is the price after the 10% discount?
一只钢笔的原价是20元，九折后的价钱是多少元．
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">20 * (9/10)</gadget>
<output>18</output>

<result>18</result>