There are 15 cows grazing on a meadow, and they can eat up all the grass in 8 days. If the first 15 cows eat for 2 days, and then 2 more cows come, then the grass can be eaten in a total of 7 days; if the first 15 cows eat for 2 days, then 5 more cows come, how much The sky can eat up the grass. Assuming the grass grows at a constant rate, each cow eats the same amount of grass every day.
一片草地，有15头牛吃草，8天可以把草全部吃光．如果起初这15头牛吃了2天后，又来了2头牛，则总共7天就可以把草吃完，如果起初这15头牛吃了2天后，又来了5头牛，则总共多少天可以把草吃完．假定草生长的速度不变，每头牛每天吃的草量相同．
<gadget id="calculator">15 * 8</gadget>
<output>120</output>

<gadget id="calculator">2 * 15</gadget>
<output>30</output>

<gadget id="calculator">17 * 5</gadget>
<output>85</output>

<gadget id="calculator">30 + 85</gadget>
<output>115</output>

<gadget id="calculator">120 - 115</gadget>
<output>5</output>

<gadget id="calculator">5 * 8</gadget>
<output>40</output>

<gadget id="calculator">120 - 40</gadget>
<output>80</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">10 - 30</gadget>
<output>-20</output>

<gadget id="calculator">80 + (-20)</gadget>
<output>60</output>

<gadget id="calculator">20 - 5</gadget>
<output>15</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

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

<result>6</result>