It takes 12 hours and 15 hours for the express train and the local train to complete the distance between A and B respectively. Now two cars leave from places A and B at the same time and go towards each other. After driving 54 kilometers, the express car found that it was missing something, and immediately started again. As a result, the two cars met at the midpoint. How many kilometers does the express train travel per hour?
快车和慢车行完A、B两地之间的路程分别需要12小时、15小时．现在两车分别从A、B两地同时开出，相向而行．快车驶出54千米后，发现少带了东西，马上重新出发，结果两车在中点相遇．快车每小时行多少千米？
<gadget id="calculator">15 / 2</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">(15/2) - 6</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">108 / (3/2)</gadget>
<output>72</output>

<result>72</result>