The number of girls in the sixth grade of Qinghe Primary School was originally boys (6/7), and then 6 girls were transferred. At this time, the ratio of girls to the whole class is 20:41. How many girls are there in the sixth grade of Qinghe Primary School?
青河小学六年级原有女生人数是男生的(6/7)，后又转来6名女生，这时女生与全班人数的比是20：41．青河小学六年级原有女生多少人？
<gadget id="calculator">20 / 21</gadget>
<output>20/21 = around 0.952381</output>

<gadget id="calculator">6 / 7</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">(20/21) - (6/7)</gadget>
<output>2/21 = around 0.095238</output>

<gadget id="calculator">6 / (2/21)</gadget>
<output>63</output>

<gadget id="calculator">63 * (6/7)</gadget>
<output>54</output>

<result>54</result>