Let assume in X years, the table will be twice as old as the sofa.
36+X=2(12+X), X=12
Or think it the other way. The difference of ages between two things never change. So they must have a difference(of age) of 24 forever. When will this difference(24) be as same as the younger's age so that the older one is exactly twice as old as the younger? 12 years later when the sofa is 24.
