The store buys a batch of pens at a price of 10 yuan each, and sells them at a price of 40% profit. When 75% of this batch of pens is sold, the profit has been 300 yuan. How many pens are there in this batch?
商店以每支10元的价格购进一批钢笔，按40%的利润定价出售，当卖出这批钢笔的75%时就已经获得利润300元，这批钢笔有多少支？
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">10 * (3/4)</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">10 * (3/4) * (2/5)</gadget>
<output>3</output>

<gadget id="calculator">3 - 10</gadget>
<output>-7</output>

<gadget id="calculator">(15/2) + (-7)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">300 / (1/2)</gadget>
<output>600</output>

<result>600</result>