Shortcut Trick
Total time gap between 3 years ago and 13 years from now = 3 + 13 = 16 years.
Ratio of A : B shifts from 5 : 7 to 9 : 11 (an exact increase of 4 parts for both A and B).
⇒ 4 parts = 16 years ⇒ 1 part = 4 years.
⇒ B's age 3 years ago = 7 parts = 7 × 4 = 28 years.
⇒ Present age of B = 28 + 3 = 31 years ⇒ Present age of C = 31 − 11 = 20 years.
∴ The correct answer is 20 years.
Alternate Method
Given:
Ratio of A and B, 3 years ago = 5 : 7
Ratio of A and B, 13 years from now = 9 : 11
Present age of C = Present age of B − 11
Calculations:
⇒ Let the ages of A and B, 3 years ago, be 5x and 7x respectively.
⇒ Present age of A = 5x + 3
⇒ Present age of B = 7x + 3
⇒ Age of A, 13 years from now = (5x + 3) + 13 = 5x + 16
⇒ Age of B, 13 years from now = (7x + 3) + 13 = 7x + 16
⇒ According to the given ratio: (5x + 16) / (7x + 16) = 9 / 11
⇒ 11 × (5x + 16) = 9 × (7x + 16)
⇒ 55x + 176 = 63x + 144
⇒ 63x − 55x = 176 − 144
⇒ 8x = 32
⇒ x = 4
⇒ Present age of B = 7(4) + 3 = 28 + 3 = 31 years
⇒ Present age of C = B's present age − 11
⇒ Present age of C = 31 − 11 = 20 years
∴ The correct answer is 20 years.
Additional Information
Ratio Difference Method
When the age difference between two periods is the same for both persons, their respective ratio parts will increase or decrease equally. This equal change in parts directly corresponds to the total time gap in years.
Cross-Multiplication Approach
If the difference in ratio parts is unequal, multiply the ratios inversely by the difference between the opposite ratio's parts to make the gap proportional, or use the basic equation method (a + t)/(b + t) = c/d.
Present Age Adjustment
Always identify the base year of your assumed variable (x). If x is defined for n years ago, the present age is derived by adding n years. If x is for the future, subtract n years to find the present age.