There is a three-digit number, after dividing by 7, 8, 9 respectively, the sum of the remainder is 21. What is the three-digit number.
有一个三位数，分别除以7，8，9后，所得的余数的和是21．这个三位数是多少．
<gadget id="calculator">7 * 8 * 9</gadget>
<output>504</output>

<gadget id="calculator">504 - 1</gadget>
<output>503</output>

<result>503</result>