The speed ratio of the passenger car and the truck is 7:5. The two cars start from two places at the same time, go towards each other, and meet at a distance of 18 km from the midpoint. How many kilometers have the passenger car traveled at this time?
客车与货车的速度比是7：5，两车同时从两地出发，相向而行，在离中点18km处相遇，这时客车行了多少千米？
<gadget id="calculator">7 + 5</gadget>
<output>12</output>

<gadget id="calculator">7 / 12</gadget>
<output>7/12 = around 0.583333</output>

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

<gadget id="calculator">(7/12) - (1/2)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">18 / (1/12)</gadget>
<output>216</output>

<gadget id="calculator">216 * (7/12)</gadget>
<output>126</output>

<result>126</result>