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?

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.