The ratio of the original number of books between A and B schools is 7:5. If school A gives 650 books to B, the ratio of books between A and B is 3:4. How many books does School A have?
甲乙两校原有图书本数的比是7：5，如果甲校给乙校650本，甲、乙两校图书本数的比就是3：4．原来甲校有图书多少本？
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<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 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(7/12) - (3/7)</gadget>
<output>13/84 = around 0.154762</output>

<gadget id="calculator">650 / (13/84)</gadget>
<output>4_200</output>

<gadget id="calculator">4_200 * (7/12)</gadget>
<output>2_450</output>

<result>2_450</result>