A piece of grass grows at a uniform rate. 10 cows can eat for 40 days, and 15 cows can eat for 20 days. How many days can 25 cows eat?
一片草地以均匀速度增长，10头牛可以吃40天，15头牛可以吃20天，那么25头牛可以吃多少天．
<gadget id="calculator">10 * 40</gadget>
<output>400</output>

<gadget id="calculator">40 * 10</gadget>
<output>400</output>

<gadget id="calculator">15 * 20</gadget>
<output>300</output>

<gadget id="calculator">400 - 300</gadget>
<output>100</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<gadget id="calculator">100 / 20</gadget>
<output>5</output>

<gadget id="calculator">5 * 40</gadget>
<output>200</output>

<gadget id="calculator">400 - 200</gadget>
<output>200</output>

<gadget id="calculator">25 - 5</gadget>
<output>20</output>

<gadget id="calculator">200 / 20</gadget>
<output>10</output>

<result>10</result>