What's the fastest way to gain weight?

Fastest like I'll see changes in a week fast! I mean I know that unrealistic but I want to put on 5 pounds at least. And for those "healthy" people I do work out but I don't want muscle I've been doing muscle work out for a while (3 times a week 4 a month) are there any other ways?
What's the fastest way to gain weight?
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