Xiaoqiang walks to school at a speed of 180 m/min, and after school at a speed of 120 m/min, what is his average speed of going back and forth in m/min?
小强上学步行的速度是180米/分，放学步行的速度是120米/分，他往返的平均速度是多少米/分．
<gadget id="calculator">1 / 180</gadget>
<output>1/180 = around 0.005556</output>

<gadget id="calculator">1 / 120</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">(1/180) + (1/120)</gadget>
<output>1/72 = around 0.013889</output>

<gadget id="calculator">2 / (1/72)</gadget>
<output>144</output>

<result>144</result>