subject to

x1 +x2 +3x3 - x4 <= 5
x1 +2x3 +x4 <= 3
y1 +y2 +3y3 - y4 <= 5
y1 +2y3 +y4 <= 3

and

x1, x2, x3, x4 >= 0
y1, y2, y3, y4 >= 0

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