sweater with a 30% discount costs 21 yuan less than a 20% discount. How much is the original price of this sweater?
一件毛衣，打七折比打八折少花21元，这件毛衣原价多少钱？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">(4/5) - (7/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">21 / (1/10)</gadget>
<output>210</output>

<result>210</result>