In the mathematics extracurricular interest group, boys accounted for (5/9) last semester. After adding 21 girls this semester, boys only accounted for (2/5). How many girls are there in this group?
数学课外兴趣小组，上学期男生占(5/9)，这学期增加21名女生后，男生就只占(2/5)了，这个小组现有女生多少人．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">21 * (2/5)</gadget>
<output>42/5 = around 8.4</output>

<gadget id="calculator">5 / 9</gadget>
<output>5/9 = around 0.555556</output>

<gadget id="calculator">(5/9) - (2/5)</gadget>
<output>7/45 = around 0.155556</output>

<gadget id="calculator">(42/5) / (7/45)</gadget>
<output>54</output>

<gadget id="calculator">1 - (5/9)</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">54 * (4/9)</gadget>
<output>24</output>

<gadget id="calculator">24 + 21</gadget>
<output>45</output>

<result>45</result>