Xiao Ming's average speed from A to B is 3 m/s, and then returns from B on the same road, with an average speed of 7 m/s. So, what is Xiao Ming's average speed back and forth?
小明从A地到B地的平均速度是3米/秒，然后从B地原路返回，平均速度是7米/秒．那么，小明来回的平均速度是多少
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/3) + (1/7)</gadget>
<output>10/21 = around 0.47619</output>

<gadget id="calculator">2 / (10/21)</gadget>
<output>21/5 = around 4.2</output>

<result>21/5 = around 4.2</result>