b) The general flow pattern of one network is shown below (see Figure 1). Assume all values are non-negative. A OUT 20 X3 B . X1 OUT X4 IN 80 X2 Figure 1: Network Flow Fill in the blank and write the SLE (AX=B) for this network. NO solution required. Vertices Flow in Flow out AX=B A B Total 80 X4 + 20 X4 = 60 = Hint: The SLE contains 4 unknown variables with 4 linear equations. (6 marks)