I'll show you now how to do it on the TI-83 or TI-84 calculator.
If you have to do it the long way, by z-score and normal curve tables,
let me know and I will show you how to do it the long way.  Be sure to
let me know if your table has negative numbers on the z-values or only
positive numbers.  


1.) Find the value of z such that approximately 30.78% of the distribution lies
between it and the mean. 2.) x has a normal disrtibution with a mean of 80.0
and a standard deviation of 3.5. Find the following probabilities. 

(A). P(x < 75.0) 

On your TI-83 or T-84, 

get this on the screen:

normalcdf(-99999999,75.0,80.0,3.5)

To get normalcdf( press 2ND STAT 2
The comma key is just above the 7 key. 

Press ENTER and read 

.0765637714




(B). P(78.0 < x < 83.0) 

On your TI-83 or T-84, 

get this on the screen:

normalcdf(78.0,83.0,80.0,3.5)

To get normalcdf( press 2ND STAT 2
The comma key is just above the 7 key. 

Press ENTER and read 

.5204625387


(C). P(x > 85.0)

On your TI-83 or T-84, 

get this on the screen:

normalcdf(85.0,99999999,80.0,3.5)

To get normalcdf( press 2ND STAT 2
The comma key is just above the 7 key. 

Press ENTER and read 

.0765637714

Edwin