It is 999999998.
999 is the greatest 3 digit number and it is odd because it ends with a 9.
The greatest 3-digit number is 999. 9+9+9 = 27 The sum of the greatest 3-digit number is 27.
The number 9 is the greatest one-digit number.
To be an even number then the final digit = 2. The largest single prime digit is 7 (as both 8 and 9 are composite). The greatest even number fulfilling the conditions is 77777772.
Since the greatest digit is 9 and the greatest 2-digit number is 99, the product of them is 891.
8642 is the greatist number if all the numbers are not the same and even
Since the greatest 1-digit number is 9, the answer is 9 x 9 = 81.
With no digit rejected, 999999999 With no digit repeated, 987654321