oldest        2y
middle         x
youngest       y
total         64

Let x be the age (in years) of the youngest sister.


Then the oldest sister is 2x years old and the middle age sister is  (2x-6) years old.


The sum is  x + 2x + (2x-6), and it is 64.


Your equation is


x + 2x + (2x-6) = 64,

5x = 64 + 6 = 70

x = 70/5 = 14.


Answer.  The youngest sister is 14 years old;  the oldest is  2*14 = 28 years;  the middle age sister is 28-6 = 22 years old.


Check.   14 + 28 + 22 = 64.    ! Correct !

     This problem is to be sold using one unknown and one equation.

Let the youngest’s age be Y

Then oldest’s is 2Y

Also, middle sister’s is: 2Y - 6 

We then get: Y + 2Y + 2Y – 6 = 64

5Y = 70

Y, or youngest is:  

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