3. What's the man's age?

A man is three times as old as his son. Five years later he will be two and a half times as old as his son.What's the man's age?

Answer: 
Age of the man = 45
Explanation:
Let the age of son & father be x & 3x.
After 5 yrs
son=x+5
father=3x+5
Also given,
2.5(x+5)=3x+5,
solving gives x=15.
fathers age=3x=45