A, B, C, and D start from the same place and all go to a certain place. A walks for a while, and then B, C, and D start together at the same time. After 6 hours, B catches up with A; after 9 hours Finally, C catches up with A; after 12 hours, D catches up with A. It is known that B travels 27 kilometers per hour, and C travels 23 kilometers per hour, so how many kilometers per hour does D travel?
甲、乙、丙、丁四人从同一地点出发都去某地，甲先走了一段时间，然后乙、丙、丁三人一起同时出发，经过6小时后，乙追上甲；经过9小时后，丙追上甲；经过12小时后，丁追上甲．己知乙每小时行27千米，丙每小时行23千米，那么丁每小时行多少千米？
<gadget id="calculator">27 * 6</gadget>
<output>162</output>

<gadget id="calculator">27 - 23</gadget>
<output>4</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

<gadget id="calculator">6 / 3</gadget>
<output>2</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">23 - 8</gadget>
<output>15</output>

<gadget id="calculator">15 * 6</gadget>
<output>90</output>

<gadget id="calculator">162 - 90</gadget>
<output>72</output>

<gadget id="calculator">15 * 12</gadget>
<output>180</output>

<gadget id="calculator">72 + 180</gadget>
<output>252</output>

<gadget id="calculator">252 / 12</gadget>
<output>21</output>

<result>21</result>