Xiao Zhang climbs the mountain, from the foot of the mountain to the top of the mountain, going up the mountain at a speed of 4 kilometers per hour, and reaches the top of the mountain. Returning back the same way at a speed of 6 kilometers per hour, how many kilometers per hour is the average speed of Xiao Zhang going back and forth?
小张爬山，从山脚到山顶，以每时4千米的速度上山，到达山顶．又以每小时6千米的速度原路返回，小张往返的平均速度是每小时行多少千米？
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/4) + (1/6)</gadget>
<output>5/12 = around 0.416667</output>

<gadget id="calculator">2 / (5/12)</gadget>
<output>24/5 = around 4.8</output>

<result>24/5 = around 4.8</result>