Red Star School organizes students to line up for an outing on foot. The walking speed is 1 meter per second. Teacher Li at the end of the line rushes to the head at a speed of 2.5 meters per second, and then returns immediately. It takes 10 minutes. What is the length of the queue?
红星学校组织学生排成队步行去郊游，步行的速度是每秒1米，队尾的李老师以每秒2.5米的速度赶到排头，然后立即返回，共用10分钟．求队伍的长度是多少？
<gadget id="calculator">600 * 2.5 * 2.5</gadget>
<output>3_750</output>

<gadget id="calculator">600 * 2.5</gadget>
<output>1_500</output>

<gadget id="calculator">1_500 - 1_500 - 600</gadget>
<output>-600</output>

<gadget id="calculator">3_750 + (-600)</gadget>
<output>3_150</output>

<gadget id="calculator">2.5 - 1</gadget>
<output>1.5</output>

<gadget id="calculator">2.5 + 1 + 1.5</gadget>
<output>5</output>

<gadget id="calculator">3_150 / 5</gadget>
<output>630</output>

<result>630</result>