The distance between A and B is 21 kilometers. A and B set off from A and B respectively at 8:00 a.m. and go toward each other. A returns immediately after arriving at B, and B also returns immediately after arriving at A. It's the second time they meet. At this time, the journey of Bank A is 9 kilometers longer than the journey of Bank B. Ask A how many kilometers per hour?
A、B两地相距21千米，上午8时整甲、乙分别从A、B两地出发，相向而行，甲到达B地后立即返回，乙到达A地后也立即返回，上午11时整他们第二次相遇．此时，甲行的路程比乙行的路程多9千米．问甲每小时行多少千米？
<gadget id="calculator">21 * 3</gadget>
<output>63</output>

<gadget id="calculator">63 + 9</gadget>
<output>72</output>

<gadget id="calculator">72 / 2</gadget>
<output>36</output>

<gadget id="calculator">36 / 3</gadget>
<output>12</output>

<result>12</result>