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To use the N, I, PMT, PV and FV keys, you must enter values in left-to-right order by pressing the appropriate keys. In later HP financial calculators, you could "randomly" key in values in any order, but the HP-80 requires you to do them left-to-right. It WAS their first financial calculator, after all. ;-)
Problem 1: If you invest $1000 into a savings account paying 10% per year, what is the balance in the account after 7 years?
Key 7 -> Press N Key 10 -> Press I Key 100 -> Press PVNow press FV. Answer should be 1948.72
Problem 2: If you need to accumulate $100,000 in 20 years, how much do you need to deposit today into a savings account paying 8% compounded quarterly?
(Since interest is added to the account every 3 months, or 4 times a year, N is 20 years times 4 periods per year or 80 quarters. Interest is 8% compounded quarterly, which is 8 divded by 4 or 2% per quarter.)
Key 80 -> Press N Key 2 -> Press I Key 100000 -> Press FVNow press PV. Answer should be $20,510.97.
Problem 3: What is the monthly payment on a house with a 8.75% interest rate, 30 year mortgage of $150,000?
(Monthly payments and 30 years means N will be 360 months. I will be 8.75 / 12 or about 0.7292% per month.)
Key 360 -> Press N Key 8.75 ENTER 12 / -> Press I Key 150000 -> Press PVNow press PMT. Answer should be $1,180.05.
Problem 4: If you deposit $100 a month into a savings account paying 6%, compounded monthly, what is the account balance after 6 years of deposits?
(Monthly deposits and 6 years means N is 72. 6% monthly is 0.5% per month).
Key 72 -> Press N. Key 6 ENTER 12 / -> Press I Key 100 -> Press PMTNow Press FV. Answer should be $8,640.89.
The HP-80 Statistics functions include being able to solve for Mean and Standard Deviation as well as Trend Line (Linear Regression). However, due to the limited memory capacity of this very early machine, several non-obvious things must be done to use them properly because the HP-80 uses the stack to store some of the intermediate summations. If you change the stack, these functions won't work.
Mean and Standard Deviation Example. Suppose you want to calculate these functions for this data set: 100, 80, 90. Here's how you would do it:
1) Press the yellow shift key and then CLX. (This clears the whole machine).
2) Then press 100 and SIGMA+, 80, SIGMA+, and 90, SIGMA+.
Do NOT press any other keys in the middle of this process.
3) To obtain the mean, press the X-Bar key.
4) To find the standard deviation (only of the X values -- Y values are NOT accumulated), you must first calculate the Mean! Then press the X <> Y key. The displayed value is the standard deviation. (It is stored in the "Y" register for those of you familiar with the RPN stack).
5) To continue with additional data points, you MUST press X <> Y again. Then you MUST also press the Gold shift key and then the X-Bar key again. (This is the function labeled ->SIGMA). This puts the HP-80 back into a "statistics" mode.
6) To correct an improperly input value, key in the incorrect value and press the gold shift key and then the SIGMA+ key.
Trend-Line (Linear Regression) Example. (Note: The HP-80 can ONLY do trend-line calculations when you have sequential time-series data with known sequential data values where each data point is 1 period after the prior data point. Also, only the Y-values are input). Suppose you have a time series of data points: (1,1), (2,2), (3,4) and want to compute the Y values in periods 4, and 5. You'd also like to know the slope of the line and the intercept.
1) Press the gold shift key and CLX to clear the entire calculator.
2) Sequentially enter the Y-values of each data point by pressing 1, TL, 2, TL, 4, TL.
3) To terminate the data-entry mode, press the gold shift key and then TL.
4) To compute a specific value along the trend line, enter the appropriate time period number and press the "n" key on the top row, then press the TL key. The displayed number is the prediction for Y. You would enter 4, n, then TL. To compute the value for period 5, press 5, n, TL.
5) If you want sequential y values along the trend line, you may just press the TL key again and again. Each sucessively displayed value is the next Y value along the trend line. If you press the X <> Y key, you can see the period number being displayed. Be sure to press X <> Y again before continuing.
6) To obtain the slope, after performing either steps 4 or 5 above, press the Roll Down key twice. The value shown is the slope. (It is stored in the "Z" register of the stack). To continue with the Trend Line, you must press the Roll Down key two more times to restore the stack to its original position.
7) Since the Y-intercept is merely the value of Y when X is 0, enter 0, n, then press the TL key. The displayed value is the intercept.