Find the ratio of ages

The following puzzle I posted to one of Orkut polls, try it.

Assume X age is K years now. X is twice as old as Y was when X was as old as Y is now. Find the age ratio of X and Y

Hint: Let A and B are age of X and Y respectively, after C years it will be such that A + C = k (B + C), where k is integer.

The answer is 4:3. How is it possible? My emphasis is on problem solving approach, not the answer.