I5 is calculated as I3 - I4 + I2 + I1, confirming the conservation of electric charge. The discussion emphasizes that current should be viewed as a flow, where the total current at a point is the sum of incoming currents minus outgoing currents. It is clarified that all currents entering a junction are considered positive, while those leaving are negative. The final value for I5 is determined to be -12A, adhering to the established rules of current flow. Understanding these principles is essential for accurate calculations in electrical circuits.