The selling price of a product is 99 yuan and 9 cents, and after the price reduction, it is 79 yuan and 9 cents and 9 cents. How much is the price reduction of this product?
一件商品的售价是99元9角，降价后是79元9角9分，这件商品降价多少元？
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">99 + (9/10)</gadget>
<output>999/10 = around 99.9</output>

<gadget id="calculator">9 / 100</gadget>
<output>9/100 = around 0.09</output>

<gadget id="calculator">79 + (9/10) + (9/100)</gadget>
<output>7_999/100 = around 79.99</output>

<gadget id="calculator">(999/10) - (7_999/100)</gadget>
<output>1_991/100 = around 19.91</output>

<result>1_991/100 = around 19.91</result>