Calculator Connundrum - Posted by Bert G in ND

Posted by Eduardo (OR) on February 08, 2001 at 12:58:42:

Hi Bert–

Yes, you are punching the numbers wrong. The HP-12C always gives the same answer as the TI. Except in one case: If you set the calculators to show all the numerals in the display to the tiniest fraction of a number, the last two or three numbers will often be different. This is because different brands of calculators use different sets of numbers internally to perform calculations (could be 8, 10, 12–usually the more expensive calculator gives the more accurate answer). However these differences are negligible for us real estate or note investors because they only amount to fractions of a penny. Otherwise, if you follow the directions for using the specific calculator (TI uses algebraic system; HP-12C uses reverse polish notation for entry) you will get the same answer whatever calculator you use. In other words: the calculator is never wrong, you are the problem if the answers don’t agree. By the way, in your example, the correct answer is $412.88 (the convention is that fractions are always rounded UP to the next highest penny in mortgage payment problems. Good luck, --Eduardo

Calculator Connundrum - Posted by Bert G in ND

Posted by Bert G in ND on February 03, 2001 at 10:42:41:

I just got my copy of “Calculator Power”, and eagerly went ripping into it with my shiney new HP12c. Imagine my horror when none of the answers matched those in the book. Could I have been punching the numbers wrong??

Then I pulled out battle-scarred TI BA-35, and the answers matched the book perfectly. Hmmmmm

Here’s an example. PV = 40,000 ; I = 11(.917) ; N = 240(20 yr)
Solve for PMT

HP12c gives 367.08
BA-35 gives 412.87

Over 20 years, that would result in a difference of nearly $11,000!! Thats over a quarter of the original principal amount.

So here’s the question. Am I doing something horribly wrong, or should one of these calculators be used as a pareweight?