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[OCR] Contact forces in a lift (mechanics)

I came across a question on a past paper involving lifts and contact forces:
http://www.ocr.org.uk/images/79221-question-paper-unit-g481-mechanics.pdf
Question 5d).
I've tried looking at the mark scheme and examiner report to understand the answer but I can't figure it out. I don't really understand what the mark scheme is saying so I'd appreciate somebody explaining how to properly answer this question and explain the physics behind it. Thanks in advance.