The round trip up and down the mountain is 420 kilometers in total. The speed of a car going up the mountain is 30 kilometers per hour, and the speed of going down the mountain is 42 kilometers per hour. How many kilometers per hour does the car travel on average?
上山下山往返路程共420千米，一辆汽车上山的时速为30千米，下山的时速为42千米，这辆汽车平均每小时行驶多少千米？
<gadget id="calculator">420 * 2</gadget>
<output>840</output>

<gadget id="calculator">420 / 30</gadget>
<output>14</output>

<gadget id="calculator">420 / 42</gadget>
<output>10</output>

<gadget id="calculator">14 + 10</gadget>
<output>24</output>

<gadget id="calculator">840 / 24</gadget>
<output>35</output>

<result>35</result>