A car is driving from Yibin to Chongqing. The road between the two places is 420km long. It has been driven for 5 hours, and there is still 30km away from Chongqing. How many kilometers per hour does the car travel on average?
一辆汽车从宜宾开往重庆，两地之间的公路长420km，已开出5小时，离重庆还有30km．汽车平均每时行多少km？
<gadget id="calculator">420 - 30</gadget>
<output>390</output>

<gadget id="calculator">390 / 5</gadget>
<output>78</output>

<result>78</result>