min 15 x1 + 10 x2 + 9 x3 + 7 x4
s.t.
x1 + x2 + x3 + x4 = 1000
x3 > 400
2 x1 + 3 x2 + 4 x3 + 5 x4 < 3300
3 x1 + 4 x2 + 5 x3 + 6 x4 < 4000