When I was 9 years old, my sister was 1/3 my age. Now my sister is 12 years old. How old am I?
You were 9 and Your sister was 1/3 of your age i.e her age was 3 years.
It means the difference between your ages was 9–3 = 6.
So now, when your sister is 12 years old then you must be 12+6 = 18 years old.
Again if you check the difference between your age and your sister's age that is 18–12 = 6.
So Your Age in this way is 18 Now.