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a+b = 20 => a = 20-b
3a+5b = 90 => 3(20-b)+5b = 90 => 60-3b+5b = 90 =>
=> 2b = 90-60 => b = 30:2 => b = 15 inecuatii
a = 20-15 = 5 ecuatii
x - ecuatiile si y - inecuatiile
x+y = 20 ⇒ x = 20-y
3x+5y = 90
3(20-y)+5y = 90
60-3y+5y = 90
2y = 90-60
y= 30/2
y= 15 inecuatii
x== 20-15
x= 5 ecuatii