Question 152304
Let the present age of Billy be 'x' and the present age of his father 'y'.
Since Billy's father is twice as old as Billy,so

y=2x

11 years ago: 
Billy's age was x-11 years.
Billy's father's age was y-11 years.
Since father was thrice as old,therefore:

y-11=3*(x-11)

Substituting for y we get,

2x-11=3*(x-11)
2x-11=3x-33
x=22

Hence, Billy is 22 years old.