Because the weather is getting colder, the grass on the pasture is decreasing at a fixed rate. It is known that the grass on the pasture can feed 20 cows for 6 days or 16 cows for 7 days. According to this calculation, how many days can the grass of this pasture last for 13 cows?
因为天气渐冷，牧场上的草以固定的速度减少．已知牧场上的草可供20头牛吃6天或可供16头牛吃7天．照此计算，这个牧场的草可供13头牛吃多少天．
<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<gadget id="calculator">16 * 7</gadget>
<output>112</output>

<gadget id="calculator">120 - 112</gadget>
<output>8</output>

<gadget id="calculator">7 - 6</gadget>
<output>1</output>

<gadget id="calculator">8 / 1</gadget>
<output>8</output>

<gadget id="calculator">8 * 6</gadget>
<output>48</output>

<gadget id="calculator">120 + 48</gadget>
<output>168</output>

<gadget id="calculator">13 + 8</gadget>
<output>21</output>

<gadget id="calculator">168 / 21</gadget>
<output>8</output>

<result>8</result>