In a group of students, 15 girls left. At this time, the ratio of boys to girls was 2:1. After that, 45 boys left, and the ratio of the remaining boys to girls was 1:5. How many girls were there?
一批学生，女生走了15名，这时男生与女生人数的比是2：1，此后，男生走了45名，余下的男生与女生人数的比是1：5，女生原来共有多少名？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 - (1/5)</gadget>
<output>9/5 = around 1.8</output>

<gadget id="calculator">45 / (9/5)</gadget>
<output>25</output>

<gadget id="calculator">25 + 15</gadget>
<output>40</output>

<result>40</result>