A pile of grass can feed 3 cows and 4 sheep for 14 days, or feed 4 cows and 15 sheep for 7 days. If this pile of grass is fed to 6 cows and 7 sheep, how many days can it last?
一堆草，可以供3头牛和4只羊吃14天，或者供4头牛和15只羊吃7天．将这堆草供给6头牛和7只羊吃，可以吃多少天．
<gadget id="calculator">3.5 * 4</gadget>
<output>14</output>

<gadget id="calculator">14 + 15</gadget>
<output>29</output>

<gadget id="calculator">3.5 * 6</gadget>
<output>21</output>

<gadget id="calculator">21 + 7</gadget>
<output>28</output>

<gadget id="calculator">7 / 28</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">29 * (1/4)</gadget>
<output>29/4 = around 7.25</output>

<result>29/4 = around 7.25</result>