Posted by Brad on Tuesday, March 20, 2012 at 10:26pm.
From a normal distribution table (example
http://www.math.unb.ca/~knight/utility/NormTble.htm
)
look up the z-value for 95% probability, which is 1.645.
μ=40 minutes, σ=7 minutes.
Calculate n from the transformation of x to z as follows:
z=(x-μ)/σ
x=σz+μ
Everything on the right hand side is know, so calculate x required.
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