The bookstore shipped a batch of books. After selling (2/5), another 120 books were shipped in. The ratio of the books at this time to the original books is 3:4. How many books were originally there?
书店运来一批图书，卖出(2/5)后，又运进120本，这时的书与原来书的比是3：4，原来有图书多少本？
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">2 / 5</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">(3/4) - (3/5)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">120 / (3/20)</gadget>
<output>800</output>

<result>800</result>