Question 1203735
Trying to follow the description literally and giving variable for each person's age, 


{{{system(j+x+m=42,m<j<x,2m=x,x-m=6,j=15)}}}

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Ignoring the inequality and doing the substitutioin for j

{{{system(15+x+m=42,2m=x,x-m=6)}}}


{{{system(x+m=42-15,x=2m,x=m+6)}}}
Last two equations should allow solving for m.


{{{2m=m+6}}}

{{{cross(m=6)}}}
From this,  {{{cross(x=12)}}}


Jess's age already was given, 15.


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Misinterpretation that {{{x-m=6}}}.  Above results may be wrong.