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.

Wheel Calculator: Merchant's Graphic Profit Finder

Wheel Calculator:  Merchant's Graphic Profit Finder

Quick Facts

Model:  Merchant's Graphic Profit Finder

Company:  G.C. Company

Years:  1954

Type:  Business and Merchandise

I purchased this wheel calculator at a swap meet in Long Beach, California in February 2023.

What is the Selling Price?

The Graphic Profit Finder is a paper slide rule calculator.   A stationary wheel sits on top of a sliding wheel.   

Red Side:  Calculating selling price based on cost and mark-up

Black Side:  Calculate selling price based on cost and margin

The stationary wheel has markup percentages and margin percentages.  

Mark-Up:  Use the red arrow to set the cost.  Set the arrow to the profit mark-up, which will point to the selling price.   

Margin:  Use the black arrow to set the cost.  Set the arrow to the margin, which will point to the selling price.

Each side has two additional arrows attached to the stationary wheel:  Cost Per Dozen and Cost Per Gross.

Formulas Used

Let x = Cost

Cost Per Dozen = 1.2 * x

Cost Per Gross = 1.44 * x

Selling Price on Mark-Up = x * (1 + mark-up% ÷ 100)

Selling Price on Margin = x ÷ (1 - margin% ÷ 100)

More sliding and wheel calculators to possibly come,

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. 

HP 12C: Percent Markup and Percent Margin

HP 12C:  Percent Markup and Percent Margin

I would like to thank Gamo for this question. 

Calculating Percent Markup and Percent Margin

Two common calculations in business is calculating the percent margin and markup.  The formulas are:

Percent Markup = (Selling Price – Cost) / Cost

Percent Margin = (Selling Price – Cost) / Selling Prices

There is no dedicated keys on the HP 12C for this, but we can use either the percent change key ([ Δ% ]) key or the TVM keys.

Using the Percent Change Key

Percent Markup:  cost [ ENTER ] price [ Δ% ]

Percent Margin:  price [ ENTER ] cost [ Δ% ] [ CHS ] (note that the arguments are in reverse order)

Example:  Cost:  25.00,  Price:  36.00

Percent Markup:  25.00 [ENTER] 36.00 [ Δ% ],  result:  44.00

Percent Margin:  36.00 [ENTER] 25.00 [ Δ% ] [CHS], result:  30.56

Using the TVM Keys

Clear the registers before you begin.  For this exercise, PMT = 0.

Percent Markup with TMV solver (HP 12C):  

Let N = 1 and PMT = 0

[ i ] = percent markup

[ PV ] = cost (enter/displayed as a negative)

[ FV ] = price

For our example:

N = 1, PV = -25, FV = 36.  Solving for i gets 44.

Percent Margin with TMV solver (HP 12C):  

Let N = 1 and PMT = 0

[ i ] = percent margin (enter/displayed as a negative)

[ PV ] = price (enter/displayed as a negative)

[ FV ] = cost

For our example:

N = 1, PV = -36, FV = 25.  Solving for i, then pressing [CHS] obtains 30.56.

What is good about the above method is that you can solve for any of the variables.

More Methods:  Solutions Handbook

Detailed algorithms for business calculations can also be found in the HP 12C Advanced Solutions Handbook (see source below).  For the Platinum edition of the Solutions Handbook, refer to page 92. 

Source:

Tony Hutchins, Luiz Vieria, and Gene Wright.  HP 12C Platinum Solutions Handbook.  Hewlett Packard Development Company, L.P.  Rev. 03.04  2004

This book can be downloaded from this site (and others) at no cost:  https://support.hp.com/us-en/product/hp-12c-platinum-financial-calculator/384706/model/315565/manuals

Eddie

This blog is property of Edward Shore, 2017.