The TAX+ and TAX- Keys

[Eddie W. Shore]

Using the TAX+ and TAX- keys for sales tax and finance calculations (present value/future value):

Link to blog entry: https://edspi31415.blogspot.com/2019/04/...ators.html

I didn't remember Hewlett Packard ever selling calculators with TAX+ and TAX- until the OfficeCalc 100 and 200 came up: http://www.hp.com/hpinfo/newsroom/press_...ators.html

I don't know if HP is still selling these models nor I don't recall ever seeing these calculators in the office stores.

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Finance: Using [ TAX+ ] to calculate Future Value

Time for a little unorthodox use of the TAX keys to calculate simple compound interest problems. If you have an investment and you want to know how much your account will be in n periods (usually year), you can use the [ TAX+ ] [ = ] combination.

FV = PV * (1 + r%)^n

FV = future value
PV = present value
r% = interest rate, stored as the TAX rate
n = number of periods

Procedure:

[ MRC ] [ MRC ]
present value [ M- ]
Loop: [ TAX+ ] [ = ] (do this n times for n periods)
(display future value)
[ M+ ] [ MRC ]
(display interest earned)

Example:

You deposit $1,000.00 in a moderate to aggressive investment account that pays an average of 9.5% per year. What is the balance after 5 years? How much interest is earned in those five years?

[ MRC] [ MRC ]
1000 [ M- ]
[ TAX+ ] [ = ]
[ TAX+ ] [ = ]
[ TAX+ ] [ = ]
[ TAX+ ] [ = ]
[ TAX+ ] [ = ] (loop the last two keys 5 times)
Future value: $1,574.24 (8-Digit display: 1574.2385)

[ M+ ] [ MRC ]
Interest earned: $574.24 (8-Digit display: 574.2385)


Finance: Using [ TAX- ] to calculate Present Value

Similarly, we can use the [ TAX- ] [ = ] combination to calculate the present value of a discounted note.

PV = FV / (1 + r%)^n

FV = future value
PV = present value
r% = interest rate, stored as the TAX rate
n = number of periods

Procedure:

[ MRC ] [ MRC ]
future value [ M+ ]
Loop: [ TAX- ] [ = ] (do this n times for n periods)
(display future value)
[ M- ] [ MRC ]
(display interest earned)

Example:

You want to save $10,000.00 in five years. You find an account that pays 9.5% annual interest. How much will you need to deposit today to get that $10,000.00 goal in five years?

[ MRC] [ MRC ]
10000 [ M+ ]
[ TAX- ] [ = ]
[ TAX- ] [ = ]
[ TAX- ] [ = ]
[ TAX- ] [ = ]
[ TAX- ] [ = ] (loop the last two keys 5 times)
Present value: $6,352.28 (8-Digit display: 6352.2775)

[ M- ] [ MRC ]
Interest earned: $3,647.72 (8-Digit display: 3647.723)

[Gamo]

I have found another ways to calculate the [Tax-] for a calculator that have [MU] function.

Very short steps and easy to remember too.

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Steps:

Price include tax [÷] Tax Rate [+/-] [MU] Answer Price without Tax [MU] Tax Amount.

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Example: Tax Rate is 9.5

The Price included tax is 480 what is the Price without Tax and Tax Amount?

480 [÷] 9.5 [+/-] [MU] display 438.36 [MU] display 41.64

Answer: Price without tax is 438.36 and 41.64 is the Tax amount.

Remark:

This [Tax-] calculation I used the Casio AX-120S set decimal digits with [5/4] and [2]


Gamo

[Gamo]

Another alteranative way to calculate [Tax-] even that calculator already have this function or not.

In this case I use Casio Model MS20-UC that alreay have [Tax-] and [Tax+] and with this funcions
can be calculate manually for when user too lazy to change the Tax Rate often so this steps is very helpful.

I'll use the same example problem from the previous post above.

Steps:

480 [+] 9.5 [+/-] [%] display 438.356164384 ≈ 438.36

[-] display 41.6438356164 ≈ 41.64

Remark:

The steps shown above work on certain model.

438.36 [x] 9.5 [%] display 41.6442 [+] display 480.0042

If your calculator calculate percentage this way most likely it will work the same.

Gamo