In a pasture, the pasture grows at the same rate every day. It is known that this pasture can feed 10 sheep for 20 days, or 15 sheep for 10 days. How many days does this piece of grass last for 30 sheep?
一片牧场，牧草每天生长的速度相同，已知这片牧草可供10头羊吃20天，或可供15头羊吃10天．那么这片牧草可供30头羊吃多少天．
<gadget id="calculator">20 * 10</gadget>
<output>200</output>

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

<gadget id="calculator">15 * 10</gadget>
<output>150</output>

<gadget id="calculator">200 - 150</gadget>
<output>50</output>

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

<gadget id="calculator">50 / 10</gadget>
<output>5</output>

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

<gadget id="calculator">200 - 100</gadget>
<output>100</output>

<gadget id="calculator">30 - 5</gadget>
<output>25</output>

<gadget id="calculator">100 / 25</gadget>
<output>4</output>

<result>4</result>