Before graduation, Xiaojun went to the store to buy gifts for his classmates. All the money he brought was bought into notebooks, which could buy 10 copies, and all the money he brought could be bought into fountain pens, which could buy 15 pieces. He first bought 4 notebooks, and then bought all the remaining money into pens. How many can he buy?
毕业前，小军到商店给同学买礼物，所带的钱全部买成笔记本，可以买10本，全部买成钢笔，可以买15只．他先买了4本笔记本，然后把剩下的钱全部买成钢笔，可以买几只？
<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>