Question

solve
x+y=180
x-3y=-20

Answer 1

x+y=180
x-3y=-20
(-) (+) (+)
---------------
4y=200
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y=200/4
y=50

x+y=180
x+50=180
x=130

Answer 2

x + y = 180
⇒ x = 180-y

x - 3y = -20
⇒ (180-y) - 3y = -20
⇒ 180 - y - 3y = -20
⇒ -4y = -20 - 180 = -200
⇒ 4y = 200
⇒ y = 200/4 = 50
x = 180 - y = 180 - 50 = 130

So x = 130, y = 50.