The digits we can use are 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
The largest 7-digit number when all digits are different is 9876543
The smallest 6-digit number when all the digits are different is 102345
We need to find the sum of 9876543 and 102345
9876543 + 102345 = 99,78,888