Shortcut Trick
Present ratio of A : B = 2 : 3
Ratio after 5 years = 3 : 4
Difference in ratio parts for both A and B = (3 − 2) = (4 − 3) = +1 part.
Since the vertical increase in parts is identical, this 1 part corresponds directly to the 5 years passed.
B's present age = 3 parts = 3 × 5 = 15 years.
∴ The correct answer is 15 years.
Alternate Method
Given:
The present ratio of ages of A and B is 2 : 3.
After 5 years, the ratio becomes 3 : 4.
Age Ratio Change Over 5 Years
Present
2
:
3
+1 unit
+1 unit
After 5 yrs
3
:
4
1 unit change = 5 years → B's present = 3 × 5 = 15 years
Calculations:
⇒ Let the present ages of A and B be 2x and 3x respectively.
⇒ After 5 years, A's age = (2x + 5)
⇒ After 5 years, B's age = (3x + 5)
⇒ The new ratio is 3 : 4, so (2x + 5) ÷ (3x + 5) = 3 ÷ 4
⇒ By cross-multiplying: 4 × (2x + 5) = 3 × (3x + 5)
⇒ 8x + 20 = 9x + 15
⇒ 9x − 8x = 20 − 15
⇒ x = 5
⇒ B's present age = 3x = 3 × 5 = 15 years.
∴ The correct answer is 15 years.
Additional Information
Ratio Difference Method
When the age of two persons increases by the same amount, their age difference remains constant. If the vertical change in ratio parts is identically equal, you can directly equate 1 part to the given number of years to find the multiplying factor.
Cross Multiplication Trick
If the initial ratio is a : b and becomes c : d after 't' years, the value of 1 part in the initial ratio can be found using the direct formula: x = [t × (c − d)] ÷ (a×d − b×c).
Constant Age Difference Principle
The numerical difference between the ages of any two individuals is always constant, whether it is calculated in the past, present, or future.