In a pasture, 17 cows can eat the grass in 30 days, 19 cows can eat the grass in 24 days, and there are 40 cows. After eating for 6 days, 4 heads were sold. How many more days will it take for the remaining cows to eat up the grass?
一牧场，17头牛30天可将草吃完，I9头牛24天可将草吃完，现有牛40头．吃6天后卖了4头．余下的牛再吃多少天便将草吃完？
<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">6 * 9</gadget>
<output>54</output>

<gadget id="calculator">40 * 6</gadget>
<output>240</output>

<gadget id="calculator">54 - 240</gadget>
<output>-186</output>

<gadget id="calculator">240 + (-186)</gadget>
<output>54</output>

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

<gadget id="calculator">54 / 27</gadget>
<output>2</output>

<result>2</result>