The original price of a storybook is 20 yuan, and now each book is sold at a 10% discount from the original price. How much is each book cheaper?
一本故事书原价20元，现在每本按原价打九折出售，每本便宜了多少元？
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">20 * (1/10)</gadget>
<output>2</output>

<result>2</result>