L = 2S - 4,   (1)
L + S = 119.  (2)

From (1), substitute L = 2S - 4 into (2), replacing L. You will get

(2S-4) + S = 119.

3S = 119 + 4 = 123.


S = 41.              (son)

L = 2*41 - 4 = 78.   (Lora)

Let son's age, be S

Then Laura's age is: 2S - 4

We then get the following SUM-OF-AGES equation: S + 2S - 4 = 119

3S = 119 + 4

3S = 123

S, or son is:  

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