The school library originally had a total of 630 music books and art books, of which music books accounted for 20%. Later, a batch of music books were bought, and now music books accounted for 30%. How many music books were bought later?
学校图书室原有音乐书和美术书共630本，其中音乐书占20%，后来又买进一批音乐书，这时音乐书占30%，后来买进多少本音乐书？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">630 * (4/5)</gadget>
<output>504</output>

<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">504 / (7/10)</gadget>
<output>720</output>

<gadget id="calculator">720 - 630</gadget>
<output>90</output>

<result>90</result>