The school bought a batch of books, 35% of which were allocated to the lower grades, and the rest were distributed to the middle and upper grades at a ratio of 7:6. It is known that the lower grades have 20 more books than the upper grades. How many books did the school buy in total?
学校买了一批图书，其中35%分给低年级，其佘的按7：6分给中、高年级，已知低年级比高年级多20本，学校一共买图书多少本？
<gadget id="calculator">35 / 100</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">1 - (7/20)</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">7 + 6</gadget>
<output>13</output>

<gadget id="calculator">6 / 13</gadget>
<output>6/13 = around 0.461538</output>

<gadget id="calculator">(13/20) * (6/13)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(7/20) - (3/10)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">20 / (1/20)</gadget>
<output>400</output>

<result>400</result>