Here, (P  -  7) + (Q  -  7) + (R  -  7) = 31
P + Q + R = 52
Q is the eldest and P is 4 years younger to R. Then,
Q > R > P
P = R  -  4
The difference between the ages of Q and R is more than 3 but less than 7. Then,
7 > Q  -  R > 3
Case 1: when Q = R + 4
Then, R  -  4 + R + 4 + R = 52
3R = 52
R = 52/3 (but present ages of P, Q and R are integers)
So, this case is invalid.
Case 2: when Q = R + 5
R  -  4 + R + 5 + R = 52
3R = 51
R = 17
Case 3: when Q = R + 6
R  -  4 + R + 6 + R = 52
3R = 50
R = 50/3 (but present ages of P, Q and R are integers)
So, this case is invalid.
So, only one possible age of R, i.e., 17 years.