37 people left the classroom, and 9 people came in. At this time, there were 20 people in the classroom. How many people were there in the classroom?
教室里走了37人，又进来9人，这时教室里有20人，教室里原来有多少人？
<gadget id="calculator">20 - 9</gadget>
<output>11</output>

<gadget id="calculator">11 + 37</gadget>
<output>48</output>

<result>48</result>