‘지내(다)’ means directly 'live', but in here it means 'doing' or 'going'. 'Verb + 고’ means 'verb + present progress'. So ‘잘 지내고 있어요?’ means 'how's going?' in formel. Without question mark : 'i'm doing fine'
And '오랜만’ is correct. It means ‘in a long time'.
October 19, 2018
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