An athlete trains to climb a mountain. Starting from point A, the road up the mountain is 30 kilometers long and travels 3 kilometers per hour. After climbing to the top of the mountain, go down the mountain along the original road, and go down the mountain at 6 kilometers per hour. What is the average speed of the athlete going up and down the hill?
一个运动员进行爬山训练．从A地出发，上山路长30千米，每小时行3千米．爬到山顶后，沿原路下山，下山每小时行6千米．这位运动员上山、下山的平均速度是多少．
<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">30 / 6</gadget>
<output>5</output>

<gadget id="calculator">10 + 5</gadget>
<output>15</output>

<gadget id="calculator">2 / 15</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">30 * (2/15)</gadget>
<output>4</output>

<result>4</result>