The ratio of the number of white rabbits to black rabbits raised in the farm is 7:4. There are 30 more white rabbits than black rabbits. How many black rabbits are there?
饲养场养的白兔与黑兔只数的比是7﹕4，白兔比黑兔多30只，黑兔有多少只．
<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>