Because the right-side is the MIDDLE OF THE CIRCUIT, for which positive-electron-flow transitions to negative-electron-flow. In DC Electrical Circuits, electricity flows from positive (+) to negative (-) ONLY. DC = Direct Current. By starting at the MIDDLE OF A DC SERIES-PARALLEL CIRCUIT or at the 'right-side', resistances can be combined sequentially and accurately.
why do you have to start with the right side?
Because the right-side is the MIDDLE OF THE CIRCUIT, for which positive-electron-flow transitions to negative-electron-flow. In DC Electrical Circuits, electricity flows from positive (+) to negative (-) ONLY. DC = Direct Current. By starting at the MIDDLE OF A DC SERIES-PARALLEL CIRCUIT or at the 'right-side', resistances can be combined sequentially and accurately.