The school library uses a sum of money to buy books. If you buy all the same kind of teaching reference books, you can buy 15 books. If you buy all the same kind of story books, you can buy 10 books. Now I only bought 4 story books, and the rest of the money will be spent on teaching reference books. How many books can I buy?
学校图书馆用一笔钱去买书，如果全部买同种教学参考书，可以买15本，如果全部买同种故事书可以买10本．现在只买了4本故事书，剩下的钱全部买教学参考书，可以买多少本．
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 4</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(3/5) / (1/15)</gadget>
<output>9</output>

<result>9</result>