Quote Originally Posted by old_toolmaker View Post
When electronic calculators first came out we didn’t trust them, so we cross checked the calculator with pencil and paper! When both agreed we were satisfied. I’m sure this sounds strange to the younger crowd, but “true story”.
There was a check for the digital calculators that involved multiplying the number 12345679 by a whole number multiple of 9 that was greater than 0 and less than 10. If the calculator gave a result that was the multiplier, supposedly the calculator was working correctly. 12345679 x 81 gave you 999999999, 12345679 x 18 gave you 222222222, etc. I doubt it checks all the functions even on a 4-function calculator, but I never ran into a calculator that didn't get the correct results. I was told that one of the spreadsheet programs, I think an early version of Excel didn't get it right, but my brain has fried a number of times since then. I barely remembered how to do this check myself. And the 2007 version does get it right.

Trust but verify!

Bill