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
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