The grass on a pasture grows at a constant rate every day. This piece of grass will feed 17 cows for 30 days, or 19 cows for 24 days. There is a herd of cows, 4 of which are sold after 6 days of eating, and the remaining cows are eaten for 2 more days. How many cows are there in this herd?
一牧场上的青草每天都匀速生长．这片青草可供17头牛吃30天，或供19头牛吃24天．现有一群牛，吃了6天后卖掉4头，余下的牛又吃了2天将草吃完，这群牛原来有多少头？
<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">17 - 9</gadget>
<output>8</output>

<gadget id="calculator">8 * 30</gadget>
<output>240</output>

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

<gadget id="calculator">240 + 72</gadget>
<output>312</output>

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

<gadget id="calculator">312 + 8</gadget>
<output>320</output>

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

<result>40</result>