Michelle asked her father how old he is. “Guess”, the father answered. “If you add the digits that make my age this year, you’ll get a number three times bigger than if you do it next year.” How old is the father?

He is 39 because 3+9=12, and next year he would be 40. 4+0=4, and 12 is 3 times larger than 4. He's old.