Xiao Ming climbs a mountain and travels 4 kilometers per hour when going up the mountain. After reaching the top of the mountain, he immediately returns to the original road and travels 6 kilometers per hour. How many kilometers per hour does Xiao Ming travel on average when he goes up and down the mountain? What is the correct column
小明爬山，上山时每小时行4千米，达到山顶后立即按原路返回，每小时行6千米，求小明上山、下山平均每小时行多少千米？正确的列式是多少
<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>