Two people, A and B, are walking toward each other from A and B at the same time. A travels 5 km per hour, and B travels 4 km per hour. After the two met for the first time, they both continued to move forward, and arrived at B and A respectively, and then returned immediately at the original speed. From the beginning to the second meeting, it took 4.5 hours. How many kilometers are the distance between A and B?
甲、乙二人分别从A，B两地同时相向而行，甲每小时行5km，乙每小时行4km．二人第一相遇后，都继续前进，分别到达B，A两地后又立即按原速度返回．从开始走到第二次相遇，共用了4.5小时，A，B两地相距多少千米？
<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">4.5 / 3</gadget>
<output>1.5</output>

<gadget id="calculator">9 * 1.5</gadget>
<output>13.5</output>

<result>13.5</result>