4 . Use the " Big M " method to reformulate the following restrictions to fit an mixed - integer programming mode a ) Either constraint 3 X 1 - * 2 - * 3 + X4 5 12 is satisfied or constraint X 1 + X 2 + X3 + X4 5 15 is satisfied b ) Only one of the constraint below must be satisfied 2x 1 + 5 * 2 - X3 + x4 5 30 X1 + 3 * 2 + 5X3 + x4 540 3 X1 - X2 + 3x3- x1 5 60