In a uniformly growing meadow, if there are 15 cows grazing, they can eat all the grass in 8 days; You can finish eating the grass. If the first 15 cows eat for 2 days, then 5 more cows come, how many days will it take to finish eating the grass?
一片均匀生长的草地，如果有15头牛吃草，那么8天可以把草全部吃完；如果起初这15头牛在草地上吃了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，再过多少天可以把草吃完？
<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>

<result>4</result>