In a mountaineering activity, Xiaogang walked up the mountain at 2.8km per hour and reached the top of the mountain at 3.5. After the event is over, Xiaogang will arrive at 2.1 hours down the mountain from the original road. In this activity, how many kilometers per hour did Xiaogang travel on average?
在一次登山活动中，小刚上山每小时走2.8km，3.5时到达山顶．活动结束后，小刚原路下山2.1时到达．在这次活动中，小刚平均每时行多少千米？
<gadget id="calculator">3.5 + 2.1</gadget>
<output>5.6</output>

<gadget id="calculator">2 / 5.6</gadget>
<output>0.357143</output>

<gadget id="calculator">2.8 * 3.5 * 0.357143</gadget>
<output>3.500001</output>

<result>3.500001</result>