The sum of three consecutive natural numbers is 21. What is the product of these three consecutive natural numbers?
三个连续自然数的和为21，这三个连续自然数的积是多少．
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<gadget id="calculator">6 * 7 * 8</gadget>
<output>336</output>

<result>336</result>