Students participating in group exercises line up. If one line is added, there will be 40 people in each line; if one line is reduced, there will be 50 people in each line.
参加团体操的同学排队，如果增加一行，每行站40人，如果减少一行，每行站50人，问：参加团体操的同学原来站多少行？
<gadget id="calculator">40 + 50</gadget>
<output>90</output>

<gadget id="calculator">50 - 40</gadget>
<output>10</output>

<gadget id="calculator">90 / 10</gadget>
<output>9</output>

<result>9</result>