The ratio of the ages of A and B, 8 years ago, was 5 ∶ 7. The ratio of their ages, 8 years from now, will be 9 ∶ 11. If the present age of C is 13 years less than that of B, and the present age of D is 8 years less than that of the age of A, then the sum of the present ages of C and D, in years, is :