Using a TI-84 calculator to find the lowest 20% of a normal curve I am not the greatest at math, I am very confused about the second part of this problem, the problem is 

Given a normal distribution of values for which the mean is 70 and the
  standard deviation is 4.5.  Find the probability that a value is
  between 65 and 80 using your TI calculator. 

Which I got was 0.8536
The second part of the questions is the troublesome part (for me at least) it asks 

Using the same normal curve above, determine the value for which the
  lowest 20% of the data is under using your TI calculator

I have attached the graph being used 
 A: Comment. Six hours and no answer to what ought to be an easy question.
You are asking for the 20th percentile or quantile .2. This is found using the inverse CDF. I do not have a TI calculator at hand, but seem to recall there is a way to find inverse CDFs. From R statistical software the answer is 16.2127 because in R the normal inverse CDF function is called 'qnorm'. (This doesn't match the colored area in the picture you linked. I think that figure is for the first part of the problem.)
qnorm(.2, 70, 4.5)
##  66.2127

You can verify this is right by verifying the probability between $-\infty$ and 
 66.2127 is 0.20. Or that the probability between $-100$ and 66.2127 is o.20.
I hope this helps. 
Maybe someone with a TI calculator will see this and tell you exactly
how to do inverse CDFs on a TI. (I edited your Question to make it easier for
the right person to find.) Also, I suppose you can google something like
TI calculator inverse normal CDF and get help on the Internet.
This Answer on our site mentions an invNORM function on a TI-84. Maybe it will help.
