The original price of a book is 20 yuan, how much is the selling price with a 10% discount?
一本书原价20元，打九折出售的价格是多少元．
<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>