school conducts military training. The entire team is 450 meters long and travels at a speed of 1.5 meters per second. A student correspondent has to rush from the end of the line to the head of the line to send notifications to the leaders and then return to the end of the line. The speed at this time is per second 3 meters, so how many seconds does it take for the correspondent to go back and forth?
某校进行军事训练，整个队伍长450米，以每秒1.5米的速度行进，一个学生通讯员要从排尾赶到排头送通知给各领队后立即返回排尾，这时的速度时每秒钟3米，那么这个通讯员往返共需多少秒钟？
<gadget id="calculator">3 - 1.5</gadget>
<output>1.5</output>

<gadget id="calculator">450 / 1.5</gadget>
<output>300</output>

<gadget id="calculator">3 + 1.5</gadget>
<output>4.5</output>

<gadget id="calculator">450 / 4.5</gadget>
<output>100</output>

<gadget id="calculator">300 + 100</gadget>
<output>400</output>

<result>400</result>