The number of red goldfish in the goldfish tank is 3 times that of black goldfish. There are 12 more red goldfish than black goldfish. How many black goldfish are there?
金鱼缸里红金鱼的条数是黑金鱼的3倍，红金鱼比黑金鱼多12条，黑金鱼有多少条？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>