What is the sum of all possible 7-digit numbers that can be

Yes, rocksailor9,

The question is likely feasible in GMAT exam, if we correlate the sequence property here.

This type of question does not follow directly the Arithmetic properties but indirectly we can apply, i.e. The difference of each side of the AP sequence have the same difference from the mean. e.g. 123, 132, 213, 231, 312, 321 has the mean as 222. Now if you find the difference between each term with respect to mean (123, 132, 213, 222, 231, 312, 321), the first and last must have same difference. Second last of each side must have same difference. The same pattern follows here also. WE CAN APPLY THE SAME PROPERTY TO N NUMBER OF DIGITS. So, we can use the formula to find sum of A.P. sequence.

Sum = n/2 (a+l)
Now, n = Total possible no. of numbers = 7p7 = n! = 7!
a= first number, i.e. smallest one = 1234567
l= last number, i.e. largest one = 7654321,

= (7!/2) (1234567+7654321)
= 7!/2 * 88888888
Ans. D.

Hope this helps.

Bunuel, kindly let me know any other better and quick approach.