Question 1077375
Start by assigning a variable to an unknown:

Let A = Ava's age

Then express other unknowns as a relation to this one (or to other assigned variables).  For each unknown, you will need to find an independent (meaning not just a previously found equation multiplied by a constant) equation. 

Let M = Ava's mother's age
We are given M=3A  (eq 1)

Let G = Ava's grandmother's age
We are given G=2M   (eq 2)

The 3rd equation comes from the given information about the sum of their ages totaling 120:
A+M+G = 120   (eq 3)

Now, (eq 3) can be written in terms of A alone, by substituting the values from (eq 1) and (eq 2).  First though we need to write (eq 2) in terms of A alone:

  G=2M, but M=3A, so G=2(3A) = 6A  (eq 2')

Now, substituting into (eq 3):
 A + M + G = 120 
 A + (3A) + (6A) = 120
  10A =  120
     A = 120/10 = 12  —>  M=36 —> G=72

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I hope this helps you understand the steps.   There are alternate methods to solving, but this problem lends itself to substitution.