There are 10 goldfish in the fish tank, 4 of them are red goldfish, and the rest are black goldfish, how many black goldfish are there?
鱼缸里有10条金鱼，其中4条是红金鱼，其余的是黑金鱼，黑金鱼有多少条．
<gadget id="calculator">10 - 4</gadget>
<output>6</output>

<result>6</result>