<PRE><B>Question 133640</B>
The average age of Amtrak passenger train cars is 19.4 years. If the distribution of ages is normal and 20% of the cars are older than 22.8 years, find the standard deviation.
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mu = 19.4
Find the z-value associated with upper 20%: 
InvNorm(0.80) = 0.8416
Use z = (x-mu)/sigma to find sigma
0.8416 = (22.8-19.4)/sigma
sigma = (22.8-19.4)/0.8416
sigma = (3.4)/0.8416 = 4.0399
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Cheers,
Stan H. 
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