The price of the commodity is 50 yuan, and it is sold at "30% off", which is how much cheaper.
商品标价50元，按"七折"出售，便宜了多少元．
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">50 * (3/10)</gadget>
<output>15</output>

<result>15</result>