The distance between A and B is 690 kilometers. A car has been driving from A to B for 4 hours, and the remaining distance is 50 kilometers longer than the already traveled one. What is the average speed of this car in km/h?
甲乙两地相距690千米，一辆汽车从甲地开往乙地，已经行了4小时，剩下的路程比已经行的多50千米．这辆汽车的平均速度是多少千米/时？
<gadget id="calculator">690 - 50</gadget>
<output>640</output>

<gadget id="calculator">640 / 2 / 4</gadget>
<output>80</output>

<result>80</result>