Maximum possible sum of 6 digits is 54 when all the digits are 9.
answer is = sum is 52 + sum is 53 + sum is 54
sum is 54 = 1(all 9s)
sum is 53 = one 8 and 5 9's = 6
sum is 52 = Four 9's and two 8's or five 9's and one 7 =
= $\binom{6}{4}$+ $\binom{6}{5}$ = 21
Ans = 1 + 6 + 21 = 28