Calculation: Fifteen years ago father's age was four times the age his son, Let the age of son be = x years ∴ Father's age = 4 × x = 4x years Now, 10 years ago father's age becomes thrice the age of his son ⇒ Son's age 10 years ago =x+5 ⇒ Father's age 10 year ago =4x+5 Now, ⇒ Father's age = 3 × age of son ⇒4x+5=3×(x+5) ⇒4x+5=3x+15 ⇒4x−3x=15−5 ⇒x=10 ⇒ Age of son 15 years ago was 10 years, at present his age is 25 ∴ After 25 years his age become 50 years.