The school queues up for exercises. If one line is added, there will be 40 people standing in each line. If there are fewer lines, there will be 50 people standing in each line. How many students do exercises in the school?
学校做操排队，如果增加一行，每行站40人，如果减少一行，每行站50人，求学校有多少学生做操？
<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>

<gadget id="calculator">9 + 1</gadget>
<output>10</output>

<gadget id="calculator">10 * 40</gadget>
<output>400</output>

<result>400</result>