There is a pasture. It is known that 27 cows are fed, and the grass is eaten up in 6 days; if 23 cows are fed, the grass is eaten up in 9 days; (Note that grass will grow)
有一片牧场，已知饲牛27头，6天把草吃尽；饲牛23头则9天把草吃尽；如果饲牛21头，牧草多少天被牛吃尽．(注意考虑牧草会生长)
<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">15 * 6</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>