A book is priced at 16.5 yuan, and you can earn 10% after the sale. How much is the purchase price of this book?
一本图书的定价为16.5元，售后可赚10%，这本图书的进价是多少元？
<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">16.5 / (11/10)</gadget>
<output>15</output>

<result>15</result>