Here's my best attempt at this one......whether it's the correct answer, I don't know.....!!!
Note that any four digit number can be written as
1000a + 100b + 10c + d
And let the sum of its digits be a + b + c + d
And we're told that
1000a + 100b + 10c + d = 109 (a + b + c + d) distribute the 109
1000a + 100b + 10c + d = 109a + 109b + 109c + 109d subtract the right side from both sides
891a - 9b -99c - 108d = 0 divide through by 9
99a - b - 11c - 12d = 0 add 12d , b to both sides
99a - 11c = 12d + b factor the left side
11 [ 9a - c ] = `12d + b divide both sides by 11
9a - c = [ 12d + b ] / 11
Note that we need to have "a" as large as possible.......
And as large as the right side can be, 10, is when d = 9 and b = 2
And this implies that a = 2 and c =8
And when the right side is 9, d = 8 and b = 3
And this implies that a = 2 and c = 9
And when the right side is 8, d = 7 and b = 4
And this implies that a = 2, but c = 10 which is impossible
So....it appears that the largest our number can be is when a = 2, b = 3, c = 9 and d = 8
So.....the number is 2398
P.S. - any corrections / additional answers by other mathematicians are welcome !!!!!!!
