There are 40 hens and 7 roosters in a chicken farm. If the number of hens is 6 times that of roosters and the number of roosters remains the same, how many hens should be added?
养鸡场有母鸡40只，公鸡7只，要使母鸡的只数是公鸡的6倍，公鸡保持不变，母鸡应当增加多少只．
<gadget id="calculator">7 * 6</gadget>
<output>42</output>

<gadget id="calculator">42 - 40</gadget>
<output>2</output>

<result>2</result>