The school bought a batch of books, 25% of which were allocated to the lower grades, and the rest were allocated to the middle and upper grades at a ratio of 7:8. It is known that the lower grades have 45 fewer books than the upper grades, how many books did the school buy in total? (solution by equation)
学校买了一批图书，其中25%分配给低年级，其余的按7：8分配给中高年级．已知低年级比高年级少45本，学校一共买图书多少本？(用方程解)
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">8 / 15</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">(3/4) * (8/15)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(2/5) - (1/4)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">45 / (3/20)</gadget>
<output>300</output>

<result>300</result>