Question 636507
let the age of the son be x
and the age of Billy be 2x
  after 10 years
 the ratio of their ages will be 3:5
 then (x+10)/(2x+10) =3/5
 or 3(2x+10)=5(x+10)
 or 6x+30=5x+50
 or 6x-5x=50-30
 or x=20 son's age
 So Billy is 2x=2*20=40 years ans