The original price of a commodity is 105 yuan, and the discount is 20% during the National Day. How much is it cheaper?
一种商品原价105元，国庆节期间八折酬宾，便宜了多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">105 * (1/5)</gadget>
<output>21</output>

<result>21</result>