A passenger car and a truck depart from cities A and B at the same time and drive opposite each other. The ratio of the speed of the passenger car to the speed of the truck is 9:7. After 3 hours of departure, the two cars meet at a distance of 25 kilometers from the midpoint of the two cities. What is the length of the highway between A and B?
一辆客车与一辆货车同时分别从A、B两城出发，相对行驶．客车速度与货车速度的比为9：7，出发3小时后，两车在距离两城中点25千米的地方相遇．A、B两城之间的公路长是多少千米？
<gadget id="calculator">25 * 2</gadget>
<output>50</output>

<gadget id="calculator">9 - 7</gadget>
<output>2</output>

<gadget id="calculator">50 / 2</gadget>
<output>25</output>

<gadget id="calculator">9 + 7</gadget>
<output>16</output>

<gadget id="calculator">25 * 16</gadget>
<output>400</output>

<result>400</result>