A piece of grassland grows evenly every day. If there are 25 cows, the grass will be eaten up in 18 days; if there are 21 cows, the grass will be eaten up in 30 days. In order to make the grass never run out, how many cows can be put on this grassland at most.
一片草地每天都均匀地长草，如果放25头牛，18天就把草地的草吃完；如果放21头牛，30天就把草吃完．为使草地的草永远吃不完，这片草地最多可以放多少牛．
<gadget id="calculator">21 * 30</gadget>
<output>630</output>

<gadget id="calculator">18 * 25</gadget>
<output>450</output>

<gadget id="calculator">630 - 450</gadget>
<output>180</output>

<gadget id="calculator">30 - 18</gadget>
<output>12</output>

<gadget id="calculator">180 / 12 / 1</gadget>
<output>15</output>

<result>15</result>