Xiao Ming went to climb a mountain on weekends. He went up the mountain at 4 km/h and went down at 5 km/h. What is his average speed for going up and down the mountain?
小明周末去爬山，他上山4千米/时，下上5千米/时，问他上下山的平均速度是多少？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">2 / (9/20)</gadget>
<output>40/9 = around 4.444444</output>

<result>40/9 = around 4.444444</result>