A father is twice his son's present age. How many years ago, was he three times his son's age, if his son's age is now 20 years?

Given that :- Present age of son = 20 years
According to question ,
father's present age = 2 × Present age of son
His father's present age = 2 x 20 = 40 years
10 years ago , The age of son = 20 ? 10 =10 years
Father's age = 40 ? 10 = 30 years = 10 × 3
Hence , required answer will be 10 years ago .