There are four cities A, B, C, and D on a section of road in sequence. AB is 30 kilometers, BC is the sum of AB and CD, and CD is half of AB plus BC. It takes 3 kilometers for a car to drive from city A to city D. What is the average speed of the car in kilometers per hour? (Note: The distance between cities A and B is referred to as AB, the distance between cities B and C is referred to as BC, and so on)
一段公路上依次有A、B、C、D四城，AB是30千米，BC是AB与CD的和，CD是AB加BC的一半，一辆汽车从A城开到D城共用了3小时，这辆汽车的平均速度是每小时多少千米？(注：A、B两城的路程简称AB，B、C两城的路程简称BC，类推)
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">60 / 1</gadget>
<output>60</output>

<gadget id="calculator">60 * 2</gadget>
<output>120</output>

<gadget id="calculator">60 + 120</gadget>
<output>180</output>

<gadget id="calculator">180 / 3</gadget>
<output>60</output>

<result>60</result>