<prompt> Human: Whats an HP-12C? Its a financial calculator released by Hewlett-Packard way back in 1981. And in the intervening 37 years or 13,302 days - you can calculate days between dates on this thing or what date will it be ___ days from today! nobody has invented a better one. These things are fast to use, useful, and built like tanks. Wall Street bankers still use them and the finance officer at your local car dealership will wilt like old spinach when you pull yours out during a negotiation. They have a ton of features that you may never use like discounted cash flow analysis, bond yield calculations, and even statistical functions. You can even write basic programs with it for multi-step calculations you need to do repeatedly. So why get one? Because the basic financial calculations are so useful to understand and easy to manage. Essentially the five keys in the top left are:n: number of periodsi: interest ratePV: present valuePMT: paymentFV: future valueAnd if you know any four of these values you can quickly find the fifth. For example: if you know what your maximum desired mortgage payment is, how long your loan will be, and what your interest rate is you can find your maximum mortgage amount. And once you have that amount you can tweak your variables and easily see what your new mortgage could be if your payment were 100 more. You can calculate how much interest you will pay over the life of a loan. You can calculate remaining balance of a loan at any given point in that loans life. You can see how much money youll have at retirement or whatever future date based on monthly contributions of X and growing at what interest rate. Im often using my HP12C while reading questions on rpersonalfinance so I figure other people posting on here might benefit as well. With a solid grasp of the time value of money concept you can handle any finance question thats likely to come your way in life!I also think our education system in the US fails a lot of people by not teaching personal finance classes. I dont want to by cynical but I think theres a lot of money to made off our financial ignorance in this country and perhaps thats why there hasnt been as big a push to teach us these things in high school. Buying an old HP-12C and then sitting down and working through the problem-solving guide is actually a great primer on these financial basics. Not only will you learn how your calculator works but you can learn some valuable information at the same time. And once you learn how to work your calculator youll be ahead of 90 of Americans in terms of financial savvy and understanding. Pro Tip: you can buy a new HP12C for 50-70 but for the same price, or even less, you can buy a vintage HP12C from eBay. Ive had a couple of new ones made in China and theyve both been lightyears behind the old 1980s made in America models in terms of build quality and construction. Heres the one I use on a daily basis. I actually returned a newer one I had just bought after I found I used this one 99.9 of the time  <response>: My mother had one of those, car dealers hated it. 