A PLA team is 450 meters long and travels at a speed of 1.5 meters per second. If a correspondent wants to convey a message from the end of the line to the head of the line, and then immediately return to the end of the line, if his speed is 3 meters per second, how much does the correspondent need to make a round trip?
一支解放军队伍长450米，以每秒1.5米的速度行进．一个通讯员要从队尾到排头去传达消息，并立即返回队尾，如果他的速度是每秒3米，那么这位通讯员往返一次共需多少．
<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>