Question 994596
b, boy
m, his mother
s, sister of boy
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MUST expect {{{b<m}}};



{{{system(b=2+2*m,b+m=9+s,s=-3+b)}}}


{{{system(2m-b=-2,m+b-s=9,b-s=3)}}}


Use the last two equations to eliminate s, and have a simpler system.
{{{-s=3-b}}}
{{{s=b-3}}}
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{{{m+b-(b-3)=9}}}
{{{m+b-b+3=9}}}
{{{m+3=9}}}
{{{highlight(m=6)}}}------?
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Use the middle equation of the system to eliminate m.
{{{m+b-s=9}}}
{{{6+b-s=9}}}
{{{b-s=6}}}---------This creates a contradiction.


This result shows b-s=6 and one of the system equations was b-s=3.  They cannot both be part of the system of equations.  One of them may be true, but then this makes the other false.


Your problem description is wrong.