X says to his son Y, "I will be of your grandfather's age when you will be of my age. Also I was of your present age when you were born." The sum of the years of all the three is 144. What is X's age?

Let son = Y, father = X, grandfather = G. X was Y's present age when Y was born: X = 2Y. When Y reaches X's age (after X-Y years), X will be G's present age: G = 2X - Y = 3Y. Y + 2Y + 3Y = 144, so Y = 24 and X = 48.