Shortcut Trick
Multiply 8 years ago ratio (2 : 3) by 2 → 4 : 6 to equalize age difference with 4 years ago ratio (5 : 7).
Change in parts = 5 − 4 = 1 unit, which corresponds to the 4 years passed.
Time gap from 4 years ago to 8 years from now is 12 years ⇒ 12 ÷ 4 = 3 units.
New ratio 8 years from now = (5 + 3) : (7 + 3) = 8 : 10 = 4 : 5.
∴ The correct answer is 4 : 5.
Alternate Method
-8 Yrs
-4 Yrs
Present
+8 Yrs
2 : 3
5 : 7
3 : 4
4 : 5
→ 4 : 6
→ 5 : 7
→ 6 : 8
→ 8 : 10
4 yrs = 1 unit
12 yrs = 3 units
Given:
Ratio of A's age to B's age 8 years ago = 2 : 3
Ratio of their ages 4 years ago = 5 : 7
Calculations:
⇒ Let the ages of A and B 8 years ago be 2x and 3x respectively.
⇒ 4 years ago, A's age = 2x + 4 and B's age = 3x + 4
⇒ According to the question, (2x + 4) / (3x + 4) = 5 / 7
⇒ 7 × (2x + 4) = 5 × (3x + 4)
⇒ 14x + 28 = 15x + 20
⇒ 15x − 14x = 28 − 20
⇒ x = 8
⇒ Present age of A = 2x + 8 = 2 × 8 + 8 = 24 years
⇒ Present age of B = 3x + 8 = 3 × 8 + 8 = 32 years
⇒ A's age 8 years from now = 24 + 8 = 32 years
⇒ B's age 8 years from now = 32 + 8 = 40 years
⇒ Required ratio = 32 : 40 = 4 : 5
∴ The correct answer is 4 : 5.
Additional Information
Constant Age Difference
The difference between the ages of two individuals always remains constant. Equalizing the horizontal difference between ratio parts simplifies age problems without requiring algebraic equations.
Cross-Multiplication Method
If initial ratio a : b changes to final ratio c : d after adding time periods x and y, you can apply the cross-product rule: (a × d) − (b × c) = (c × y) − (d × x) to solve directly.
Ratio Properties
Multiplying or dividing both terms of a ratio by the same non-zero number does not change its value. A duplicate ratio of a : b is given as a2 : b2.