math
posted by tmouery .
The population mean length of a hospital stay for surgery is 5.5 days and the standard deviation is 1.9 days. What percentage of patients are hospitalized for 3.9 days or less?
I've read the book and looked at the explanation that it gives and it's not making any sense.

3.9 days is 2.05 std from the mean.
Look that up in your Z table.
Just use the left tail value. 
What does that mean? What's a Z table and what does std mean?

http://en.wikipedia.org/wiki/Standard_normal_table
http://www.mathworks.com/help/matlab/ref/std.html
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