1) Let's pick the 2nd equation and to eliminate the x variable, multiply by -2 then add both equations like this
2x+ 4y=10
-2X+ -2y=-30
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2y = -20
y = -10
2) Now let's plug that y=-10 into one equation, usually the simpler one.
x +y = 15
x -10 = 15 add 10 to both sides
X = 15 +10
x=25
3) So the solution is S ={25, -10}
