The students lined up in a 450-meter-long team to visit the science and technology exhibition, traveling at a speed of 1.5 meters per second. On the way, Mr. Zhang chased from the end of the line to the head at a speed of 3 meters per second due to something, and then immediately returned from the head to the end of the line at the same speed. How long did it take Mr. Zhang?
同学们排成一列长450米的队伍去参观科技展览，以每秒1.5米的速度行进．途中张老师因有事用每秒3米的速度从队伍的末尾追到排头，又用同样的速度立即从队伍的排头回到末尾．张老师一共用了多长时间？
<gadget id="calculator">3 - 1.5</gadget>
<output>1.5</output>

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

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