Please tell me if my working for the following question is correct. Also, more importantly, I have some gaps in my math foundation, so please tell me what math topics I should revise/relearn for the questions I wrote in red. Thanks.
(I don't understand why + is used. Please tell me what topic I need to revise to fully understand this part)
(Why do I need to multiply this?)
Edit: The question should be I edited the original question above. Thanks TutorMeNate