When resistors are connected in series, the current through each resistor is the same. In other words, the current is the same at all points in a series circuit. When resistors are connected in series ...
You can avoid solving simultaneous equations in multiple unknowns by identifying series and parallel combinations of resistors.
Physics and Python stuff. Most of the videos here are either adapted from class lectures or solving physics problems. I really like to use numerical calculations without all the fancy programming ...