My reasoning is:
If there were none left over in each case, the answer would be the Lowest Common Multiple of 7,8,9 = 504
Since there are some left over, the answer will be a bit more or a bit less than 504 . . the question tells us that it is 500 or less
The answer must be even number . .
By trial and error I started at 500, and was prepared to go 498,496 etc
But 500 was . . BINGO!