Question 1167859
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Assume that the readings at freezing on a batch of thermometers are normally distributed with a mean of 0°C 
and a standard deviation of 1.00°C. A single thermometer is randomly selected and tested. 
Find P81, the 81-percentile. This is the temperature reading separating the bottom 81% from the top 19%.
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<pre>
To solve the problem, we should translate this set of words into meaningful human form.


We are given the normal curve with the mean value of 0 and the standard deviation of 1.


They want you find the score 'x' (the temperature) such that the area under this normal curve 
on the left of 'x' be 0.81.


Use the standard function invNorm of a regular hand calculator TI-84/87.

Its format is

    x = invNorm(area, mean, SD).


So, in this problem we want to calculate  invNorm(0.81, 0, 0.81).


The calculator gives the  <U>ANSWER</U>  x = 0.8779.


It means that 87.79% of readings will be lower than 0.81 °C.


Alternatively, you may use the Excel function NORM.INV(area,mean,SD) 
or online free of charge calculator at this web-page

https://onlinestatbook.com/2/calculators/inverse_normal_dist.html


This online calculator has an advantage showing you visually the area under the curve,
so you will learn the subject momentarily and better than if 5 or 15 teachers/tutors
will explain it to you.

With this online calculator, you will catch the problem instantly.
</pre>

Solved.