During the National Day holiday, a third-grade student of Hope Primary School travels from city A to city B, rents a bus, travels an average of 84 kilometers per hour, and arrives in 6 hours. When I came back, due to the large number of vehicles, it took 7 hours. When returning, how many kilometers per hour does the bus travel on average?
十一假期期间，希望小学三年级同学从A城到B城旅游，租用一辆客车，平均每小时行84千米，6小时到达．回来时，由于车辆多，用了7小时．回来时，客车平均每小时行多少千米？
<gadget id="calculator">6 / 7</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">84 * (6/7)</gadget>
<output>72</output>

<result>72</result>