The convoy transported a batch of relief materials to the disaster area, traveled 80km per hour, and reached the disaster area in 5 hours. It took 3 hours to return to the original road. Find the average speed of this convoy back and forth.
车队向灾区运送一批救灾物资，去时每小时行80km，5小时到达灾区．原路返回用了3小时，求这支车队来回的平均速度．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">2 / 8</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">80 * 5 * (1/4)</gadget>
<output>100</output>

<result>100</result>