Question 174508
a.   X + Y=6  OR X=6-Y
NOW REPLACE X IN THE SECOND EQUATION BY (6-Y) & SOLVE FOR Y.
2X + Y = 8 
2(6-Y)+Y=8
12-2Y+Y=8
-Y=8-12
-Y=-4
Y=4 ans. 
X=6-4
X=2 ans.
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b.7X + 3Y = 14 MULTIPLY BY -3 & ADD TO THE SECOND EQUATION
5X + 9Y = 10
-21X-9Y=-42
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-16X=-32
X=-32/-16
X=2 ans.
5*2+9Y=10
10+9Y=10
9Y=10-10
9Y=0
Y=0 ans.
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c.  4X + Y = 16 OR Y=16-4X
NOW REPLACE Y WITH (16-4X) IN THE SECOND EQUATION & SOLVE FOR X.
2X + 3Y = 24
2X+3(16-4X)=24
2X+48-12X=24
-10X=24-48
-10X=-24
X=-24/-10
X=2.4 ans.
4*2.4+Y=16
9.6+Y=16
Y=16-9.6
Y=6.4 ans.
d.  12X + Y = 25 OR Y=25-12X NOW REPLACE Y WITH (25-12X) IN THE SECOND EQUATION & SOLVE FOR X.
8X - 2Y = 14
8X-2(25-12X)=14
8X-50+24X=14
32X=14+50
32X=64
X=64/32
X=2 ans.
12*2+Y=25
24+Y=25
Y=25-24
Y=1 ans.