Question 1173550
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Let x be the Eric's age; y be the Ethan's age.  Then   


    x + y = 79

    x - y =  7


is the system of equations you are asking for.


The solution is  {{{(79+7)/2}}} = 43 for Eric's age and  {{{(79-7)/2}}} = 36 for Ethan's age.
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Solved.