Arun and Barun's Age Ratio

Let Barun's present age be $100$. Then Arun's present age is $40$. Let $x$ years pass. Arun's age = $40 + x$, Barun's age = $100 + x$. Given: $40 + x = \frac{1}{2}(100 + x)$ $80 + 2x = 100 + x \implies x = 20$. Barun's age increases from $100$ to $120$, which is an increase of $20\%$.