A father's age is four times that of his son. After 5 years, it will be three times that of his son. How many more years will take if father's age is to be twice that of his son?

 Let the present age of son = x yrs

Father’s present age = 4x

 After five years , age of son = x+5

Age of father = 4x+5

According to question,

 4x+5 = 3(x+5)

 4x+5 = 3x + 15

 4x-3x = 15-5

 x = 10

hence the present age of son  = 10 yrs

present age of father = 4*10 = 40 yrs

no , if father’s age = 2(son’s age)

 let after p years father’s age is twice of son’s age

then 40+p = 2(10+p)

  40 + p = 20+2p

  40-20 = 2p-p

  P = 20

Hence , after 20 years father’s age is twice of son’s age