A pile of grass can feed 3 cows and 5 sheep for 15 days, or 5 cows and 6 sheep for 10 days. Then how many days can this pile of grass last for 4 cows and 18 sheep? (Each cow has the same amount of food, and each sheep has the same amount of food)
一堆草，可供3头牛和5只羊吃15天，或者5头牛和6只羊吃10天．那么这堆草可供4头牛18只羊吃多少天．(每头牛的食量相同，每只羊的食量也相同)
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">9 + 5</gadget>
<output>14</output>

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

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

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

<gadget id="calculator">210 / 30</gadget>
<output>7</output>

<result>7</result>