The areas of the first, second, and third pastures are 3 hectares, 5 hectares, and 7 hectares in turn. The grass on the three pastures grows equally densely and grows equally fast. There are two herds of cows. The first herd eats the grass of the No. 1 pasture in 2 days, and the grass of the No. 2 pasture in 5 days. During these 7 days, the second group of cows just finished eating the grass on the third pasture. If the first herd has 15 cows, how many cows are there in the second herd?
第一、二、三号牧场的面积依次为3公顷、5公顷、7公顷，三个牧场上的草长得一样密，且生长得一样快．有两群牛，第一群牛2天将一号牧场的草吃完，又用5天将二号牧场的草吃完．在这7天里，第二群牛刚好将三号牧场的草吃完．如果第一群牛有15头，那么第二群牛有多少头？
<gadget id="calculator">0.5 / 4</gadget>
<output>0.125</output>

<gadget id="calculator">7 * 7 * 0.125</gadget>
<output>6.125</output>

<gadget id="calculator">6.125 + 7</gadget>
<output>13.125</output>

<gadget id="calculator">13.125 / 7 / 0.125</gadget>
<output>15</output>

<result>15</result>