car drives from the county town to Xishan Township, traveling 40 kilometers per hour when going there, and traveling 60 kilometers per hour when returning. What is the correct formula for finding the average round trip speed
一辆汽车从县城开往西山乡，去时每小时行40千米，返回时每小时行60千米．求往返的平均速度的正确算式是多少
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/40) + (1/60)</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">2 / (1/24)</gadget>
<output>48</output>

<result>48</result>