Here’s the best way to understand how to figure this out.
Go back to Unit 3, to the video at the end about the heat generated by the loose connection.
It’s basically the same scenario as the question you are working on – two loads in series, and you want to find the heat generated by one of them.
As you go through the video, do the calculation that we show on a piece of paper, making sure you understand each step. If you don’t – ask me for more help.
If you can follow that, then you should be able to do a similar calculation for the question you are asking about.