UPSC CAPF AC 2020 Paper

Given, Present age of B = 9 yr
∴ Age of B before 2 yr = 7 yr
∴ Age of A before 2 yr = 7 × 3 = 21 yr
Present age of A = 23 yr.
Let after x yr the age of A will be double of the age of B, then
⇒ 23 + x = 2(9+x)
⇒ 23 + x = 18 + 2x
∴ x = 5 yr.