If 3 cows eat all the grass on 2 acres in 2 weeks, and 2 cows eat all the grass on 2 acres in 4 weeks, how many cows can eat all the grass on 6 acres in 6 weeks? (Grass has the same height and same growth rate)
3头牛在两星期内吃完2亩地上的所有草，而2头牛在四星期内吃完2亩上的所有草，多少头牛能在六星期内吃完六亩地上的所有草？(草高度相同，生长速度相同)
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">2 / 2</gadget>
<output>1</output>

<gadget id="calculator">3 * 1 * 6</gadget>
<output>18</output>

<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

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

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

<gadget id="calculator">30 / 6</gadget>
<output>5</output>

<result>5</result>