You're right. I think I just misunderstood final tests or my coffee wasn't strong enough this morning. Anyways, sorry for wasting your time.

In my suggestion I explained algorithm, not it's implementation. I don't care about actual output and what you're going to check in final tests.

Currently for input father's age = 30, son's age = 0 correct answer is 0.
In my suggestion correct answer would be 30 (father will be 60 when his son will be 30). In my second example I explained case "from the past" - currently father is 53 and son is 27, but a year ago father was twice as old as his son.

Please, explain why you think my suggestion isn't good enough. Both father and son are aging simultaneously - that's why I thought it would be better to consider it in this kata.