987654
It is 987654321.
98
If repeated digits are allowed, then the largest 12-digit number is 999,999,999,999 .If repeated digits are not allowed, then the largest 12-digit number is 989,898,989,898 .If the same digit can't be used more than once, then the largest possible number has only10 digits. The number is 9,876,543,210 .
A palindrome reads the same forward and in reverse. This tells me that at leastthe first digit and the last digit must be the same. So it's not possible to have a6-digit palindrome "with no same digits".The largest 6-digit palindrome, with just enough repetition of digits to make it apalindrome and no more, would be 987,789 .
9876
2
99,998,765
999,987,654
99,998,765
9998888
Any pair of digits (not including 0), can be used to generate 14 four-digit numbers. If one of the digits is 0, only seven will start with a non-zero digit.
There are 900 of them.