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)