The 20% discount on "Fun Mathematics" is 12 yuan, which is much cheaper than the original price.
《趣味数学》打八折是12元，比原价便宜多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">12 / (4/5)</gadget>
<output>15</output>

<gadget id="calculator">15 - 12</gadget>
<output>3</output>

<result>3</result>