In a certain school, the number of boys was equal to the number of girls (5/7). Later, 30 boys were transferred and 10 girls were transferred away. At this time, the ratio of the number of boys to the number of girls was 9:10. How many girls are there in this school?
某校原来男生人数是女生人数的(5/7)，后来，又转来了30名男生，转走了10名女生，这时男生人数与女生人数的比为9：10．这个学校现有女生多少人？
<gadget id="calculator">9 / 10</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(9/10) * 10</gadget>
<output>9</output>

<gadget id="calculator">30 + 9</gadget>
<output>39</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(9/10) - (5/7)</gadget>
<output>13/70 = around 0.185714</output>

<gadget id="calculator">39 / (13/70)</gadget>
<output>210</output>

<gadget id="calculator">210 - 10</gadget>
<output>200</output>

<result>200</result>