Given the age of each of three sisters when laid together, would give 40. What is the age of each sister when you know that the eldest sister is as twice as old as the youngest sister, and the number 4 added to the youngest age will give us the age of the sister in-between?

Let us say the age of the youngest sister is x then age of girl in-between and eldest are x+4 and 2x respectively.
and x+ (x+4) +2x = 40 => x=9

So Age of Youngest is 9, and age of girl in-between the youngest and eldest is 13.

-- James.

Yes the answer is the youngest one is 9 years old, the one in the middle is 13, and the eldest is 18 because let the youngest one be equal to x. Then the one in the middle is equal to x+4 and the eldest is equal to 2x. Given that

x+(x+4)+2x=40
x+x+4+2x=40
x+x+2x=40-4
4x=36
4x/4=36/4
x=9