There is a pasture that is full of pasture, and the pasture grows at a uniform rate every day. This pasture can feed 17 cows for 30 days, and can feed 19 cows for 24 days. Some of the existing cows are grazing. After 6 days, 4 cows died, and the rest The cow has eaten the grass for 2 days, so how many cows are there?
有一牧场长满牧草，每天牧草匀速生长，这个牧场可供17头牛吃30天，可供19头牛吃24天，现有牛若干头在吃草，6天后，4头牛死亡，余下的牛吃了2天将草吃完，那么原来有牛多少头．
<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">9 * 30</gadget>
<output>270</output>

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

<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">240 - 24</gadget>
<output>216</output>

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

<gadget id="calculator">216 / 8</gadget>
<output>27</output>

<gadget id="calculator">27 + 4 + 9</gadget>
<output>40</output>

<result>40</result>