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3/4
There are 8 positions that elevator can be in when you get to the 4th floor. It's equally likely that it will be in any one of those 8 positions.
It's headed down in 2 of those positions (5 headed down, and 4 headed down). In the other 6, it will be headed up next time it gets to the 4th floor. Hence the chance is 6/8 = 3/4.
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Strewth
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