Shortcut Trick
Sum of ages of A, B, and C 5 years ago = Present sum − (3 persons × 5 years) = 135 − 15 = 120.
Total ratio parts 5 years ago = 4 + 5 + 7 = 16 parts.
Value of 1 ratio part = 120 ÷ 16 = 7.5 years.
Sum of ages of B and C 5 years ago = (5 + 7) parts = 12 × 7.5 = 90 years.
Sum of B and C 3 years from now = Sum 5 years ago + (5 years + 3 years) × 2 persons = 90 + 16 = 106 years.
∴ The correct answer is 106 years.
Alternate Method
Ages 5 Years Ago (Total = 16x)
A: 4x
B: 5x
C: 7x
Sum 5 years ago = Present Sum − (3 × 5) = 135 − 15 = 120
16x = 120 → x = 7.5
Target: B + C (+3 yrs) = 12x + 16 = 106
Given:
⇒ Ratio of ages of A, B, and C (5 years ago) = 4 : 5 : 7
⇒ Sum of their present ages = 135 years
Formula Used:
⇒ Sum of ages n years ago = Present sum − (n × number of persons)
⇒ Future age = Past age + Total years passed
Calculations:
⇒ Sum of their ages 5 years ago = 135 − (3 × 5) = 120 years.
⇒ Let the ages of A, B, and C 5 years ago be 4x, 5x, and 7x respectively.
⇒ Total sum 5 years ago = 4x + 5x + 7x = 16x
⇒ 16x = 120
⇒ x = 120 ÷ 16 = 7.5
⇒ Sum of ages of B and C 5 years ago = 5x + 7x = 12x
⇒ 12x = 12 × 7.5 = 90 years.
⇒ The time gap between '5 years ago' and '3 years from now' is (5 + 3) = 8 years.
⇒ Since 8 years pass for both B and C individually, their sum increases by 8 × 2 = 16 years.
⇒ Sum of ages of B and C (3 years from now) = 90 + 16 = 106 years.
∴ The correct answer is 106 years.
Additional Information
Constant Age Difference
The age difference between two individuals always remains constant, regardless of how many years pass into the past or the future.
Sum of Ages Over Time
If the sum of present ages of n people is S, their combined sum t years ago was S − (n × t), and t years hence will be S + (n × t).
Cross-Multiplication Method for Ages
When past and future age ratios are given, setting up a fraction (ax + t) ÷ (bx + t) = c ÷ d directly helps find the common multiple, where t represents the time gap.