The speed ratio of the passenger car and the truck is 7:4. The two cars start from two places at the same time, go towards each other, and meet at a distance of 18 kilometers from the midpoint. How many kilometers has the passenger car traveled at this time?
客车与货车的速度比是7：4，两车同时从两地出发，相向而行，在离中点18千米处相遇，这时客车行了多少千米？
<gadget id="calculator">4 + 7</gadget>
<output>11</output>

<gadget id="calculator">7 / 11</gadget>
<output>7/11 = around 0.636364</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/11) - (1/2)</gadget>
<output>3/22 = around 0.136364</output>

<gadget id="calculator">18 / (3/22)</gadget>
<output>132</output>

<gadget id="calculator">132 * (7/11)</gadget>
<output>84</output>

<result>84</result>