The ratio of books between A and B is 7:5. After library A gives 650 books to library B, the ratio of books between A and B is 3:4. How many books does library A have?
甲、乙两个图书馆图书之比为7：5，甲馆给乙馆650册后，甲、乙两个图书的比为3：4，原来甲馆有图书多少本．
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">(7/12) - (4/7)</gadget>
<output>1/84 = around 0.011905</output>

<gadget id="calculator">650 / (1/84)</gadget>
<output>54_600</output>

<gadget id="calculator">54_600 * (7/12)</gadget>
<output>31_850</output>

<result>31_850</result>