In a meadow, the amount of grass grows the same every day, 17 cows can eat the grass in 30 days, and 19 cows can eat the grass in 24 days a year. After eating several cows for 6 days, sell 4 cows, and eat the remaining cows for 2 days to finish the grass. How many cows were there in total?
一片草地，草每天生长量相同，17头牛30天可将草吃完，19头年24天可将草吃完．现有若干头牛吃了6天后，卖掉4头牛，余下的牛再吃2天将草吃完．原来共有多少头牛．
<gadget id="calculator">2 * 4</gadget>
<output>8</output>

<gadget id="calculator">17 * 30</gadget>
<output>510</output>

<gadget id="calculator">19 * 24</gadget>
<output>456</output>

<gadget id="calculator">510 - 456</gadget>
<output>54</output>

<gadget id="calculator">30 - 24</gadget>
<output>6</output>

<gadget id="calculator">54 / 6</gadget>
<output>9</output>

<gadget id="calculator">9 * 30</gadget>
<output>270</output>

<gadget id="calculator">510 - 270</gadget>
<output>240</output>

<gadget id="calculator">8 * 9</gadget>
<output>72</output>

<gadget id="calculator">8 + 240 + 72</gadget>
<output>320</output>

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

<gadget id="calculator">320 / 8</gadget>
<output>40</output>

<result>40</result>