Question 673833
x+y+z=31
y=x+1
x+y=z+7 
x+y-z=7 now add to the first equation.
x+y+z=31
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2x+2y=38 divide by 2
x+y=19 
y=19-x
y=-x+19 add to the second equation
y=x+1
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2y=20
y=20/2
y=10 ans.
x+10=19
x=19-10
x=9 ans.
10+9=z+7
z=10+9-7
z=19-7=12 ans.
Proof:
10+9+12=31
31=31