The original price of a book is 45 yuan. After the 20% discount, how much is it cheaper than the original price?
一本书原价45元，打八折后，比原来便宜了多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">45 * (4/5)</gadget>
<output>36</output>

<gadget id="calculator">45 - 36</gadget>
<output>9</output>

<result>9</result>