Oh yeah, that does sound like fun! Meanwhile, I have a 74 average in 10th grade general geometry. 

Aw, you're doing all right then. You're talking to someone who failed two EE courses last year. But now I'm averaging an A+ in both of them so far.   It's never impossible to improve.

It all started in fourth grade. When they didn't teach me how to subtract. And then in fifth grade when they didn't teach me how to long divide. I've been horrible at math ever since. 

I got an HP 12C today for $8.75. Is this one of those rare expensive HP's y'all talk about?  At any rate, I have no clue how to use it.

Aw sweet! The HP-12C is actually the most common HP calculator. It's been in continuous production since 1981, and is still being made today. But still, that's awesome! If you can't tell, the 12C is a financial calculator (the most popular financial calculator in the world, actually), so no scientific functions. You still get basic calculator functionality though, and programmability. Some people have written programs that allow it to compute some scientific functions like the trigonometric functions.

What's the serial number on the back? That might help determine when it was made. Does it turn on at least?

Yeah it turns on and works! I tried the equation you mentioned earlier, and it gave me 20. The serial no. is 3448S04210.

1994! So still based on the original 1981 design, I think. Does it use three button cell batteries or one or two coin batteries? And where was it made?

3 button cells, Singapore. This calculator brand new is $57.99! I got a good deal, yes?

Ah, the original design then. A good deal indeed!

Some time in the late 1990's they upgraded the power source to use a single CR2032 coin cell, and then in 2009 they completely revamped the electronics and upgraded the power source again to two coin cells. The current version is electronically completely different from the original, but works the same. While the original version has standard arithmetic circuitry like most calculators, the current version uses a 32 MHz ARM processor, and can make calculations over 100 times faster than the original (which runs at a blazing 0.2 MHz). But the original is best! They were made in the USA and several other countries, but the ones made in Singapore are reportedly as good as the ones made in the USA. The ones made today are made in China, and poor in quality compared to the originals. So even though you got the most common HP calculator, you still got a gem.  

Interestingly, the reason HP took 28 years to upgrade the speed of the 12C was because professionals stated they wouldn't trust a fast calculator as much as a slower one.

I just did some reading, and you should be able to run my pi calculation program on this. If you're up to it, I can tell you how to put in and run the program. It's really easy.

Sure, just tell me what buttons to push. I figured out how to use exponents and radicals on this calculator. Aside from basic four function math, that's about all I can do.

All right. Uh? I obviously have never actually programmed a 10C series calculator, so I'm just going to assume how it works based on how my 33E works.

Note that I'll state some functions, such as PSE and GTO, which are printed in blue or yellow. To execute those functions you have to press the appropriate shift key (f or g) first.

First go into the programming mode by pressing P/R (which is f, R/S). Now press PRGM (f, R down) to clear any program still stored in it. Now you can start entering the program. Just press the buttons listed here. You don't press ENTER or anything after each line; just enter each key after the other. Note that some lines have more than one key, like STO + 1. In that case just press STO, +, and 1. Simple as that. For GTO 07, it's GTO, 0, and 7. The display will show random stuff which you can ignore as you enter the program.

1
STO 1
4
*
PSE
RCL 1
2
+
STO 2
1/x
STO - 1
RCL 1
4
*
PSE
RCL 2
2
+
STO 2
1/x
STO + 1
RCL 1
4
*
PSE
RCL 2
GTO 07

When you're done, go back into the run mode by pressing P/R. Now set the calculator to the beginning of the program by pressing PRGM. Now press R/S to start running the program. Hopefully it works.

This is what I got after doing all that stuff.

It looks like after entering the program, you accidentally didn't press f before pressing R/S to go execute P/R and go back into the run mode. You gotta restart and enter the program again.   But it looks like up to that point you did do everything right.

Oh, and before pressing R/S to run the program, press f, 9. This will set the calculator to show everything with 9 decimal places instead of 4.

It gave me the same thing again. I promise I did it right!

Damn. Why is it pretending you're not pressing f when you try to press P/R? (Assuming that's what it's doing - it seems to be based on the display it shows.) Your f key obviously works because you're able to go into the programming mode.

Okay, lets do a super simple program. Turn the calculator off and back on to go into the run mode. Go into the programming mode, clear the program with PRGM (f, R down), then enter the following:

4
ENTER
5
*

Then press P/R (f, R/S) to go back into the run mode, press PRGM to reset to the beginning of the program, then press R/S. You should get 20. If you don't, I'd like to see a video to witness exactly what you do and what it does from start to finish.