The original price of a pair of leather shoes is 200 yuan, and now they are sold at a discount of 10% because of a promotion. How much is the price of these shoes cheaper than the original price? (List equations to solve)
一双皮鞋原价200元，现因促销打九折出售，现在卖这双皮鞋比原价便宜多少元？(列方程来解答)
<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">200 * (9/10)</gadget>
<output>180</output>

<gadget id="calculator">200 - 180</gadget>
<output>20</output>

<result>20</result>