Thanks for asking questions in the Forum!
#15: Remember what you learned in Fundamentals (Mod. 3, unit 5): the rule of thumb for equivalent resistance of loads in parallel is that it will be something less than the smallest resistance. If you run the calculation using the actual formula shown in Unit 5 of Basic Electricity in the Fundamentals course, you get about 5.5 ohms.
#18: data transfer is discussed in about the last 10 minutes of the video. There’s a slide called “serial data communications” near the end.
Let me know if you have any other questions!