Solution:For I:
Let the present age of 'P' = x years
Present age of 'R' = 2x years
Sum of the present ages of 'Q' and 'R'
= 36 × 2 = 72 years
Present age of 'Q' = (72 − 2x) years
Present age of 'S' = [(72 − 2x)/6] × 13 years
After eight years, the age of 'P' = (x + 8) years
After eight years, the age of 'T' = [(x + 8)/7] × 13 years
Present age of 'T' = [(x + 8)/7] × 13 − 8 years
Sum of the ages of 'P', 'S' and 'T' = (40 × 5) − 72
= 128 years
x + {[(72 − 2x)/6] × 13} + {[(x + 8)/7] × 13 − 8} = 128
x + (936 − 26x)/6 + (13x + 104)/7 − 8 = 128
42x + 6552 − 182x + 78x + 624 − 336 = 128 × 42
62x = 1464
x = 1464/62
Therefore, I cannot be true.
For II:
Let the present age of 'P' = x years
Present age of 'R' = 2x years
Sum of the present ages of 'Q' and 'R' = 32 × 2 = 64 years
Present age of 'Q' = (64 − 2x) years
Present age of 'S' = [(64 − 2x)/6] × 13 years
After eight years, the age of 'P' = (x + 8) years
After eight years, the age of 'T' = [(x + 8)/7] × 13 years
Present age of 'T' = [(x + 8)/7] × 13 − 8 years
Sum of the ages of 'P', 'S' and 'T' = (36 × 5) − 64
= 116 years
x + {[(64 − 2x)/6] × 13} + {[(x + 8)/7] × 13 − 8} = 116
x + (832 − 26x)/6 + (13x + 104)/7 − 8 = 116
42x + 5824 − 182x + 78x + 624 − 336 = 116 × 42
62x = 1240
x = 20
Present age of 'T' = [(x + 8)/7] × 13 − 8 = 44 years
Present age of 'S' = [(64 − 2x)/6] × 13 = 52 years
Difference = 52 − 44 = 8 years
Therefore, II can be true.
For III:
Let the present age of 'P' = x years
Present age of 'R' = 2x years
Sum of the present ages of 'Q' and 'R' = 20 × 2 = 40 years
Present age of 'Q' = (40 − 2x) years
Present age of 'S' = [(40 − 2x)/6] × 13 years
After eight years, the age of 'P' = (x + 8) years
After eight years, the age of 'T' = [(x + 8)/7] × 13 years
Present age of 'T' = [(x + 8)/7] × 13 − 8 years
Sum of the ages of 'P', 'S' and 'T' = (30 × 5) − 40
= 110 years
x + {[(40 − 2x)/6] × 13} + {[(x + 8)/7] × 13 − 8} = 110
x + (520 − 26x)/6 + (13x + 104)/7 − 8 = 110
42x + 3640 − 182x + 78x + 624 − 336 = 110 × 42
−62x = 692
x = −692/62
Age cannot be negative.
Therefore, III is false.