Question 1073788
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Let the answer be a+b+c = x

Then we have this system:

{{{system(matrix(3,9,

 a,""+"",b,""+"",c,"",""="","", x,
5a,""-"",7b,""+"",3c,"",""="","", 19,
2a,""-"",4b,""+"",c,"",""="","", 77))}}}

Eliminate the c's from the first and third equations by 
multiplying the first equation by -1 and adding it 
to the third equation:

{{{matrix(2,9,

-a,""-"",b,""-"",c,"",""="","", -x,

2a,""-"",4b,""+"",c,"",""="","", 77)}}}

We get

{{{matrix(1,7,

 a,""-"",5b,"",""="","", 77-x)}}}

Eliminate the c's from the first and second equations 
by multiplying the first equation by -3 and adding it 
to the second equation:

{{{matrix(2,9,

-3a,""-"",3b,""-"",3c,"",""="","", -3x,

5a,""-"",7b,""+"",3c,"",""="","", 19)}}}

We get

{{{matrix(1,7,

2a,""-"",10b,"",""="","", 19-3x)}}}


Next we have these two equations with c eliminated:

{{{matrix(2,7,

 a,""-"",5b,"",""="","", 77-x,
2a,""-"",10b,"",""="","", 19-3x

)}}}

Eliminate the left side by multiplying the first
equation through by -2:

{{{matrix(2,7,

-2a,""+"",10b,"",""="","", -154+2x,
2a,""-"",10b,"",""="","", 19-3x

)}}}

Adding the equations we get:

{{{matrix(1,5,

0,"",""="","",-135-x)}}}

or 

{{{matrix(1,5,

x,"",""="","",-135)}}}

Answer a+b+c = x = -135

Edwin</pre><b></font>