Aarav's father is 30 years older than Aarav. Twelve years from now, the father will be twice as old as Aarav will be then. Find Aarav's present age.

Let Aarav be $x$ years and his father $y$ years. $y-x=30$. Twelve years from now: $y+12=2(x+12)$, that is $y-2x=12$. Subtracting the second from the first gives $x=18$, so $y=48$. Check: 48 - 18 = 30, and $60=2\times 30$. 48 years answers for the father instead of Aarav; 30 years is only the gap between them; 24 years comes from adding 12 to the father's age alone and forgetting to age Aarav too.