The ratio of the original female students in the classroom to the total number of students in the classroom is 3:7. When 6 female students come in, the female students account for the total number of existing students (5/9). How many students are there in the classroom? ?
教室里原有女同学与教室里的总人数的比是3：7，当又进来6名女同学后，女同学占现有学生总人数的(5/9)，教室里原有学生多少人？
<gadget id="calculator">9 - 5</gadget>
<output>4</output>

<gadget id="calculator">5 / 4</gadget>
<output>5/4 = around 1.25</output>

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

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(5/4) - (3/4)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">6 / (1/2)</gadget>
<output>12</output>

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

<gadget id="calculator">12 / (4/7)</gadget>
<output>21</output>

<result>21</result>