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Maximize f = 240x1 + 104x2 + 60x3 + 19x4
subject to
20x1 + 9x2 + 6x3 + x4 ≤ 20
10x1 + 4x2 + 2x3 + x4 ≤ 10
xi ≥ 0, i = 1 to 4
Find all the basic feasible solutions of the problem and identify the optimal solution.