math
posted by Adam .
Al is twice as old as Bill, who is twice as old as Carl. In 10 years, Al will be twice as old as Carl. How old is Al now?
A. 40
B. 20
C. 10
D. 5

I usually start with the smallest quantity, which here is Carl's age
let Carl's age be x
then Bill is 2x
and Al is 4x
in 10 years:
Carl  x+10
Bill  2x+10
Al  4x + 10
so 4x+10 = 2(x+10)
4x + 10 = 2x + 20
2x = 10
x = 5
So now Al is 4(5) = 20
check:
present ages:
Carl  5
Bill  10
Al  20
10 years from now:
Al will be 30 and Carl will be 15
Is Al twice as old as Carl ?? YES 
thanks