A car delivers goods in a mountainous area. It goes up the mountain at 20 kilometers per hour, and goes down the mountain for the same distance at 30 kilometers per hour. How many kilometers per hour does the car go up and down the mountain on average?
一辆汽车在山区送货，上山每小时行20千米，下山行同样长的路程，每小时行30千米，这辆汽车上、下山平均每小时行多少千米？
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">2 / (1/12)</gadget>
<output>24</output>

<result>24</result>