The MU Key on a Four Function Calculator and Programs for the DM42/HP 42S

 The MU Key on a Four Function Calculator and Programs for the DM42/HP 42S

Not too long ago, I purchased this very colorful, four function calculator from the University I work at, the Dexin BST DX-818.

The DX-818 is powered by a solar panel with an option to be run by an AAA battery when using the calculator in less than bright light conditions. The AAA battery is optional.

A Standard Calculator

This calculator is the standard four function key calculator, complete with the square root function, percent function, the standard three memory keys (M+, M-, and MRC), and a grand total (GT) key. A grand total is started when calculations are completed by pressing the equals key [ = ]. The grand total is both recalled and cleared when the [ GT ] is pressed.

The Markup Equation

Before we head to the calculators, the mark up is a calculation of between selling price (new), cost (old), and mark up. The markup is ratio between the selling price and cost over cost and as expressed as a percentage (%):

MU = (selling price – cost) ÷ cost = (selling price ÷ cost - 1)

Solving for the other variables yield:

selling price = cost * (1 + MU)

cost = selling price ÷ (1 + MU)

In these equations, MU is expressed as a decimal.

If let the selling price be the new value and cost be the old value, then we have the equation for percent change (Δ%):

Δ% = (new – old) ÷ old = (new ÷ old - 1)

Similarly:

new = old * (1 + Δ%)

old = new ÷ (1 + Δ%)

In these equations, Δ% is expressed as a decimal.

The Markup Key (MU)

On this particular calculator, there is an [ MU ] key. This is the markup key. Apparently, there are two types of [ MU ] key, one set of syntax for Casio calculators with the [ MU ] key. An example of a Casio calculator with an [ MU ] key is the MJ-120D plus. They way Casio calculators use the [ MU ] key is a calculation:

cost [ MU ] markup percentage [ % ] returns selling price.

Immediately pressing [ = ] returns selling price – cost.

However, some calculators, like the colorful calculator featured (I really wish I knew the model number) with the [ MU ] have another syntax, and that is what I will address from here on out.

On this particular calculator, the syntax, using one of the four arithmetic functions is:

y < +, -, ×, or ÷ > x [ MU ]

No use of the equals key is needed, unless you want to add the result to the grand total.

The four calculations with the [ MU ] key, on this particular calculator (and possibly the NewYes calculators from China) are:

Key Sequence	Formula Used	Notes
Y [ + ] X [ MU ]	100 * (Y ÷ X + 1)
Y [ - ] X [ MU ]	100 * (Y ÷ X - 1)	Calculates markup or percent change (Δ%). Y = selling price, new value X = cost, old value
Y [ × ] X [ MU ]	Y * (1 + X ÷ 100)	Adds X% to Y. Calculates the new value (selling price) given old value (cost) (Y) and markup (X).
Y [ ÷ ] X [ MU ]	Y ÷ (1 – X ÷ 100)	Finds the original retail price given selling price (Y) and discount rate (X). This can also be used for gross-up calculations.

Sample Calculations

Problem	Keystrokes	Result
Determine 100 * (Y ÷ X + 1) with Y = 420, X = 25	420 [ + ] 25 [ MU ]	1780
An item that cost the company $14.75 sells for $19.99. Find the markup.	19.99 [ - ] 14.75 [ MU ]	35.5254237288 (≈35.53%)
A company manufactures a product with a unit cost of $32.84 and wants a 20% markup. What is the selling price?	32.84 [ × ] 20 [ MU ]	39.408 (about $39.41)
A product is currently on sale for $39.99. If the discount is 35%, what is the original retail price.	39.99 [ ÷ ] 35 [ MU ]	61.523076923 (about $61.52)

RPN Programming with the Swiss Micros DM42

Here are four RPN programs, which are were programmed using a Swiss Micros DM42, which can be used on the HP 42S (and probably the HP 41C).

Program MU+: Y [ + ] X [ MU ]

00 {17-Byte Prgm}

01 LBL “MU+”
02 ÷

03 1

04 +

05 100

06 ×

07 RTN

08 END

Program MU-: Y [ - ] X [ MU ]

00 {17-Byte Prgm}

01 LBL “MU-”
02 ÷

03 1

04 -

05 100

06 ×

07 RTN

08 END

Program MUMLT: Y [ × ] X [ MU ]

00 {12-Byte Prgm}

01 LBL “MUMLT”
02 %

03 +

04 RTN

05 END

Program MUDIV: Y [ ÷ ] X [ MU ]

00 {21-Byte Prgm}

01 LBL “MUDIV”
02 ENTER

03 100

04 ÷

05 1

06 X<>Y

07 -

08 ÷

09 RTN

10 END

Sources

 Casio. “MJ-120D Plus” https://www.casio.com/intl/basic-calculators/product.MJ-120DPLUS/ Accessed May 23, 2026.

Eaton, Glenn. “The Calculator MU Key” Eaton Family Website. Posted on June 30, 2017. https://eatonfamily.au/calculator-mu-key/ Accessed May 23, 2026.

“How to use the MU button on a calculator? #1” YouTube Short: @newyes1162. Posted December 5, 2022. https://www.youtube.com/shorts/OiS8yFKKJSU Accessed May 23, 2026

Eddie

All original content copyright, © 2011-2026. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.

Review: NY-991MS Ewriter Calculator Review

 Review: NY-991MS Ewriter Calculator Review (LINGSFIRE)

Quick Facts

Model: NY-991MS

Company: NewYes/LINGSFIRE

Price:  About $18 - $25 (purchased on Amazon)

Power:  Solar with battery (CR2032)

Display:  Two lines, numerical and fractions.  Numbers can have up to 10 digits. 

Calculator with an Electronic Writing Board

The NY-991MS is one of a new breed of calculators to come with a mini electronic writing board attached.   For this calculator, the pen is stored in a lower left-hand corner compartment.   There is a lock switch at the top of the calculator.  If the lock switch is on the left position, the board is unlocked, therefore the board to be erased with the press of a clear button.  If the lock switch is on the right, the board is locked and nothing can be erased.   

It is the writing board that got me interested in buying this calculator and is a neat feature to write quick notes.  So far, the NY-991MS is the most advanced calculator that has this feature.  

Other models include (not an all inclusive list):

JEOEUS JEOEUS-005:  basic four function calculator

LINGSFIRE:  similar to a Casio fx-300 MS

Laelr:  similar to a low-cost scientific calculator Sharp EL-501XBGR

On my calculator, the pen was hard to get out (but locks in nicely), so I keep a pen in a separate bag near this calculator.   

I love the hard folding case that the NY-991 MS comes in.  Not only it looks professional, the case is hard and protects both the calculator and the writing tablet.  I also love how large the numbers and functions are on the screen.

Not Quite A Casio fx-115MS

Now let's talk about the functionality.  I would place the NY-991 MS somewhere between the Casio fx-300MS and the Casio fx-115MS.   

Let's get the ugly out of the way.  The manual is terrible.   The manual has examples of functions that are not even present, mislabels keys, and has a lot of spelling errors.   If this is someone's first calculator, then the person is going to have a rough experience if they tried to follow the manual.   Put in some effort and proofread, NewYes.  

The NY-991MS has four main modes:

0:  COUNT.   This is the regular operating mode of the calculator. 

1:  STAT:  One-variable statistics

2:  REG:  Regression

3:  CMPLX:  Complex Number arithmetic

COUNT MODE

While a lot of functions are self-explanatory, some need to be pointed out due to some idiosyncrasies.  

The angle setting is a toggle button:

[ D|R ]: switch between degrees and radians.  There is a "G" indicator too, but the NY-991MS does not have a gradian mode.

[ 2ndF ] (D←→R):  switches between degrees and radians, plus converts the angle in the display.

When the calculator is in degrees mode, there is a "TY" indicator on the screen.  I have no idea what TY stands for nor does the manual explain this indicator.  

The number format has a three-way toggle button:

[ F/S ] (number):  Fixed mode

[ F/S ] [ F/S ] (number):  Scientific mode

[ F/S ] [ F/S ] [ F/S ]:  Float mode

The [ DMS ] key is dual purpose:  entering degree, minute, or second markers during calculations.  After the equals key is pressed, the key switches the number between degrees-minutes-seconds and decimal degrees format.   

The key sequence [ 2ndF ] ( ←→DEG) does not do anything.

The logarithm keys operate differently from most scientific calculators:

[ log ]:  takes the logarithm of any base.  The syntax:  log(number,base)

[ lg ]:  common logarithm

[ ln ] : natural logarithm

The polar/rectangular conversions do not accept parenthesis or a syntax error occurs.

OK:

5,3 [ →rθ ] returns  r (stored in X) returns = 5.830951895 and θ (stored in Y) returns 30.96375653

(5,3) [ →rθ ] gives an error

Similar with the →xy function.    I thought this key was broken or useless until I figured it out by accident.  The manual does not show the correct way to use the polar/rectangular conversion.

The key [ A^m_n ] is the permutation key.  No idea why this is labeled A.   The more appropriately labeled C^m_n is the combination key.

We can store expressions for later evaluation.   Press [ 2ndF ] (LRN) to store the expression.  Press [ COMP ] to evaluate it.  You will be prompted for the variables.  The stored expression is erased when the calculator is turned off.  There is no solve feature.  

Calculus functions:

Derivative:  d/dx( function of X, value)

Definite Integral:  ∫( function of X, lower limit, upper limit)

There is this weird arrangement of variables.  A, B, C, D are in one line while X, Y, and Z are on the 2nd row.  In this mode we have a bug:  whenever a value is stored in A, the display returns 0.   The value is retained in A though.

Want to enter numbers in scientific notation?  Press [ 2ndF ] ( Exp ) to get the E.

STAT

One variable stats are entered using the [ DATA ] button.  What I like about the statistics mode is that parameters are on the keyboard, that can be accessed by the [ ALPHA ] key.   However, s_x does NOT return the sample deviation, but the population deviation.  Likewise, (s_x)^2 returns the population variance.  

Population Deviation

REG

There are six regressions, which the variables are mislabeled in the manual.  Here are the true regression equations used:

1.  Lin:  Linear Regression,  y = a*x + b

2.  Quad:  Quadratic Regression, y = a*x^2 + b*x + c

3.  Inv:  Inverse Regression, y = a/x + b

4.  Pwr:  Power Regression, y = b * x^a

5.  Exp:  Exponential Regression, y = b * e^(a*x)

6.  Log:  Logarithmic Regression, y = a*ln x + b

In essence, other than quadratic regression, the variable a is the slope and the variable b is the y-intercept. 

The [ ALPHA ] key is needed to use the prediction functions x-hat and y-hat.  

CMPLX

The complex number mode is, as expected, limited.  Allowable operations are the arithmetic functions, the reciprocal, and the square functions.  Enter the imaginary part with the [ i ] button.    

Results are shown on part at a time between the real and imaginary part.   Press [ 2ndF ] [ i ] (←.→) to switch between the real and imaginary parts.  There is another key that is exclusive to this mode:

[ |R| ]: absolute value.   This operates only after the calculation is completed by pressing the equals key.  

[ 2ndF ] (ARG):  argument.  Likewise, this operates only after the calculation is completed by pressing the equals key.

The manual mentions a conjugate function, which isn't present on the calculator.  

Final Thoughts

This calculator had potential and unfortunately, with the arrangement of keys, mislabeling of the deviation and variance, the unusual syntax of some functions, and the horrible manual, I cannot recommend this calculator unless you are an expert user.   I was able to figure out the functions due to the fact I had a Casio calculator to compare results.   The electronic board and the beautiful folding case, while very nice, may not be enough to make up for the faults for some.  Not recommended for a beginner at all, especially if the user has to depend on the manual to use it.  

Eddie 

All original content copyright, © 2011-2023.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.