A technical book is priced at 13 yuan, and a 30% profit can be obtained after the book is sold. What is the cost of the book?
一本科技书售价13元，这本书售出后可以获得30%的利润，这本书的成本是多少．
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 + (3/10)</gadget>
<output>13/10 = around 1.3</output>

<gadget id="calculator">13 / (13/10)</gadget>
<output>10</output>

<result>10</result>