There is a pastoral area covered with grass, which grows at a constant rate every day. The grass in this pasture can feed 27 cows for 6 weeks, or 23 cows for 9 weeks, so how many weeks can it feed 21 cows?
有一牧区长满牧草，牧草每天匀速生长．这个牧区的草可供27头牛吃6周，或供23头牛吃9周，那么可供21头牛吃多少周．
<gadget id="calculator">27 * 6</gadget>
<output>162</output>

<gadget id="calculator">23 * 9</gadget>
<output>207</output>

<gadget id="calculator">207 - 162</gadget>
<output>45</output>

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

<gadget id="calculator">45 / 3</gadget>
<output>15</output>

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

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

<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">72 / 6</gadget>
<output>12</output>

<result>12</result>