There is a pasture where the grass grows evenly every day, which can feed 8 cows for 20 days or 12 cows for 10 days. If only 6 cows are eating at the beginning, a few more cows will be added from the ninth day. Tian ate all the grass, how many cows were added?
有一牧场，草每天均匀生长，可供8头牛吃20天或可供12头牛吃10天，如果开始只有6头牛在吃，从第九天开始又增加了若干头牛，在用四天吃光所有的草，问增加了几头牛？
<gadget id="calculator">12 * 10</gadget>
<output>120</output>

<gadget id="calculator">20 * 8</gadget>
<output>160</output>

<gadget id="calculator">160 - 120</gadget>
<output>40</output>

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

<gadget id="calculator">40 / 10</gadget>
<output>4</output>

<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">120 - 40</gadget>
<output>80</output>

<gadget id="calculator">4 * 4</gadget>
<output>16</output>

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

<gadget id="calculator">16 - 48</gadget>
<output>-32</output>

<gadget id="calculator">80 + (-32) + 16</gadget>
<output>64</output>

<gadget id="calculator">64 / 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 6</gadget>
<output>10</output>

<result>10</result>