The number of girls in the sixth grade of a certain school accounts for the total number of students (7/12), and later 15 girls are transferred to the school. At this time, the number of girls accounts for the total number of students (3/5), so what is the original number of girls in the sixth grade of a certain school?
某校六年级女生人数占总人数的(7/12)，后来又转进15名女生，这时女生人数占总人数的(3/5)，则某校六年级女生人数原有多少人．
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 - (3/5)</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(3/5) / (2/5)</gadget>
<output>3/2 = around 1.5</output>

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

<gadget id="calculator">1 - (7/12)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">(7/12) / (5/12)</gadget>
<output>7/5 = around 1.4</output>

<gadget id="calculator">(3/2) - (7/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">15 / (1/10)</gadget>
<output>150</output>

<gadget id="calculator">150 * (7/5)</gadget>
<output>210</output>

<result>210</result>