The Radio Shack EC-4042 has the exact same layout as the TI-30X.
The position of the graphically represented keys can be found by moving your mouse on top of the graphic.

Turn your calculator on
Press The On C Key: Upper Right Corner.
Clearing the memory
Turn the calculator off, then on again.  (That is, press The Off Key: Row 1, Column 5 The On C Key: Upper Right Corner.)

 

Entering data
one variable
Enter the first number, then press The Sigma Plus Key: Row 4, Column 1.  The screen will now say n=1 indicating that you have entered one data point.  Enter the second number, press The Sigma Plus Key: Row 4, Column 1.  The screen will now say  n=2 because you have entered two data points.   Continue until all the data has been entered.
two variables
The TI-30X does not do regression.

 

Calculating one-variable statistics
mean (x)
Press The Second Key: Row 1, Column 1 It's Yellow The X-squared Key: Row 3, Column 3 (you should see x above the key).
standard deviation for populations (s or sn)
Press The Second Key: Row 1, Column 1 It's Yellow The Divided By Key: Row 3, Column 5 (you should see sxn above the division sign).
standard deviation for samples (s or sn-1)
Press The Second Key: Row 1, Column 1 It's Yellow The Square Root Key: Row 3 , Column 4 (you should see sxn-1above the square root sign).

Calculating two-variable statistics
r (correlation)
The TI-30X does not do regression.
regression coefficients
slope
The TI-30X does not do regression.
y-intercept
The TI-30X does not do regression.


Calculating combinations and permutations
combinations (nCr)
Enter the n value.  Press The Second Key: Row 1, Column 1 It's Yellow The Pi Key: Row 3, Column 1 (you should see double arrows above the pi key), then the r value. Then press The Second Key: Row 1, Column 1 It's Yellow The Eight Key: Row 5, Column 3 (you should see nCr over the 8 key).

Curiously, there is a second way. Enter the n value. Press The Divided By Key: Row 3, Column 5. Enter the r value. Press The Second Key: Row 1, Column 1 It's Yellow The Eight Key: Row 5, Column 3 The Equals Key: Row 8, Column 5 . We have no idea why this works.
permutations (nPr)
Enter the n value.  Press The Second Key: Row 1, Column 1 It's Yellow The Pi Key: Row 3, Column 1 (you should see double arrows above the pi key), then the r value. Then press The Second Key: Row 1, Column 1 It's Yellow The Nine Key: Row 5, Column 4 (you should see nPr over the 9 key). 

Curiously, there is a second way. Enter the n value. Press The Times Key: Row 4, Column 5. Enter the r value. Press The Second Key: Row 1, Column 1 It's Yellow The Nine Key: Row 5, Column 4 The Equals Key: Row 8, Column 5 . We have no idea why this works.

 

Turning the calculator off
Press The Off Key: Row 1 Column 5.

 

Worked Out Examples

In the following examples, we list the exact key sequence used to find the answer.  We will list the keys by the main symbol on the key.  In parentheses, we will list a helpful mnemonic, e.g. we will list ex as (ex).

A: What is the mean and standard deviation of the following list of numbers?

15      16      20      21

1: Clear Memory  The Off Key: Row 1 Column 5 The ON C Key: Upper Right Corner
2: Enter Data   The One Key: Row 7, Column 2 The Five Key: Row 6, Column 3 The Sigma Plus Key: Row 4, Column 1 The One Key: Row 7, Column 2 The Six Key: Row 6, Column 4 The Sigma Plus Key: Row 4, Column 1
 The Two Key: Row 7, Column 3 The Zero Key: Row 8, Column 2 The Sigma Plus Key: Row 4, Column 1 The Two Key: Row 7, Column 3 The One Key: Row 7, Column 2 The Sigma Plus Key: Row 4, Column 1 
3: Compute the mean  The Second Key: Row 1, Column 1 It's Yellow The X-squared Key: Row 3, Column 3 (x)
4: Compute the standard deviation (population)  The Second Key: Row 1, Column 1 It's Yellow The Divided By Key: Row 3, Column 5 (sxn)
5: Compute the standard deviation (sample)  The Second Key: Row 1, Column 1 It's Yellow The Square Root Key: Row 3 , Column 4 (sxn-1)


     You should get a mean of 18, population standard deviation of 2.549509757 and a sample standard deviation of 2.943920289.

C: Find 10C6 and 9P5.
1: Compute 10C6   The One Key: Row 7, Column 2 The Zero Key: Row 8, Column 2 The Second Key: Row 1, Column 1 It's Yellow The Pi Key: Row 3, Column 1 The Six Key: Row 6, Column 4 The Second Key: Row 1, Column 1 It's Yellow The Eight Key: Row 5, Column 3 (nCr)
2: Compute 9P5  The Nine Key: Row 5, Column 4 The Second Key: Row 1, Column 1 It's Yellow The Pi Key: Row 3, Column 1 The Five Key: Row 6, Column 3 The Second Key: Row 1, Column 1 It's Yellow The Nine Key: Row 5, Column 4 (nPr)


You should get 10C6 = 210 and 9P5= 15120.

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