In a meadow, the grass grows evenly. If 25 sheep are grazing, the grass will be eaten in 6 days; if 18 sheep are grazing, the grass will be eaten in 20 days. How many sheep can be grazed at most if the grass in the pasture can never be eaten up.
一片草地，草均匀地生长，如果放牧25只羊，6天把草吃完；如果放牧18只羊，20天把草吃完．要使草地的草永远吃不完，最多能放牧多少只羊．
<gadget id="calculator">18 * 20</gadget>
<output>360</output>

<gadget id="calculator">25 * 6</gadget>
<output>150</output>

<gadget id="calculator">360 - 150</gadget>
<output>210</output>

<gadget id="calculator">20 - 6</gadget>
<output>14</output>

<gadget id="calculator">210 / 14 / 1</gadget>
<output>15</output>

<result>15</result>