a father is 45 years old and his son is 9 years old, in how many years time will the father be as old as the son?
If you think about the question you have posted you will realize that it does not make any sense. Is it possible for a son and a father to have the same age?
As the son grows older so does the father. The age difference between the two is 45 - 9 = 36 years. It will always remain the same.
The father is never as old as the son.
The dad will never be as old as the son, the dad is always going to be older. Unless you mean when is the son going to reach his father's current aga which then would be in 36 years.
I think you may have miss-typed the problem. If you meant "...in how many years will the son be as old as the father...", there are two possible answers.
If this question is taken literally, the father is currently 45-9=36 years older than the son, and will always be 36 years older.The son will never be as old as the father, at the same time.
If you assume they want to know how long it will be before the son reaches his father's current age, you use the same equation as above: 45-9=36. But now your answer is in 36 years, at which time the son will be 45, the current age of his father.