The distance between A and B is 21 kilometers. A and B set off from A and B respectively at 9:00 am and walk toward each other. A returns immediately after arriving at B, and B returns immediately after arriving at A. At 12:00 noon, they arrive at The second encounter. At this time, the distance traveled by A is 9 kilometers longer than the distance traveled by B. How many kilometers does A walk per hour?
A、B两地相距21千米，上午9时甲、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地后立即返回，中午12时他们第二次相遇．此时甲走的路程比乙走的路程多9千米．甲每小时走多少千米．
<gadget id="calculator">63 - 9</gadget>
<output>54</output>

<gadget id="calculator">54 / 2</gadget>
<output>27</output>

<gadget id="calculator">63 - 27</gadget>
<output>36</output>

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

<result>12</result>