Xiao Ming's average speed from point A to point B is 3 m/s, and then returns from point B to point A at a speed of 7 m/s on the same route, then the average speed of Xiao Ming traveling back and forth between point A and point B How many meters per second should be.
小明从A地到B地的平均速度为3米/秒，然后又从B地按原路以7米/秒速度返回A地，那么小明在A地与B地之间行一个来回的平均速度应为多少米/秒．
<gadget id="calculator">1 + 1</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>