The ratio of the number of roosters to hens raised by Mrs. Zhao is 4:7, and there are 30 fewer roosters than hens. How many roosters does Aunt Zhao's family raise?
赵大娘家养的公鸡与母鸡只数的比是4：7，公鸡比母鸡少30只．赵大娘家养的公鸡有多少只？
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<result>40</result>