The library bought 2,000 new books, of which literature and art books accounted for 30%, and after buying another batch of literature and art books, literature and art books accounted for 50% of the total. How many literature and art books were bought later?
图书馆新买进2000本图书，其中文艺书占30%，又买进一批文艺书后文艺书就占总数的50%，后来又买进多少本文艺书？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">2_000 * (7/10)</gadget>
<output>1_400</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">1_400 / (1/2)</gadget>
<output>2_800</output>

<gadget id="calculator">2_800 - 2_000</gadget>
<output>800</output>

<result>800</result>