A certain book is sold at a price of 10% lower than the original price. The second edition of the book has increased in length and is sold at a price of 9.9 yuan. What is the price of each copy of the original book?
某书出售是比原价降低了10%，第二次增订出版增加了篇幅，比上次售价增加了10%出售，售价为9.9元，问原版书每本的定价是多少元？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 + (1/10)</gadget>
<output>11/10 = around 1.1</output>

<gadget id="calculator">9.9 / (11/10)</gadget>
<output>9</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

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

<result>10</result>