The original price of a storybook is 20 yuan, and now each book is sold at 90% of the original price. How much is the current price?
一本故事书原价20元，现在每本按原价90%出售．现价多少元？
<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>