Ages of P, Q and R are in the ratio 4 : 7 : 9. 8 years ago, the age of R was Q's present age. What is the sumof their ages at present?

The present age of A, B, and C are in the ratio of 4 : 7 : 9

let present ages be x, so their  ages are A's age 4x, B's age 7x and C’s age 9x

Eight years ago, the sum of their ages was 56

It means during 8 years the sum of age 8 × 3 = 24 years to be added to obtain sum of their present ages

so the present sum of ages = 56 + 24 = 80 years. 

4x + 7x + 9x = 80

20x = 80

$$\frac{80}{20}$$

x = 4

A's age 4x = 4 × 4 = 16 years

B's age 7x = 7 × 4 = 28 years

C's age 9x = 9 × 4 = 36 years

sum of the ages = 16 + 28 + 36 = 80 years