The 90th percentile of x is 70.
What is standard deviation?
A set of values' variance or dispersion is measured by the standard deviation. While a high standard deviation implies that the values are dispersed over a wider range, a low standard deviation shows that the values tend to be close to the mean of the collection.
Given: Mean = 80, s = 5.
Computing values to standard deviation from mean,
Lower limit = mean - 2s
= 80 - 2(5)
= 80 - 10
Lower limit = 70
Hence, if x has a normal distribution with mean 80 and standard deviation 5 then the 90th percentile of x is 70.
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