A sweater is sold at 640 yuan after an 80% discount. If it is 25% off, how much cheaper will it be?
一件毛衣八打折后出售是640元，如果打七五折的话，还会便宜多少钱？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">640 / (4/5)</gadget>
<output>800</output>

<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">800 * (3/4)</gadget>
<output>600</output>

<gadget id="calculator">640 - 600</gadget>
<output>40</output>

<result>40</result>