I found this device the other day - it seems like a calculator, but the results it's giving me don't make any sense. Here, if I try this...
$6 + 3 = 38$
See! It doesn't make any sense at all. Should be nine. And this should be seven, right?
$2 - -5 = 37$
Nope. ... Both thirty something huh. It isn't always. I'll try again.
$4 \times -3 = 17$
And once more...
$4 \times -9 = 5$
I think this thing is broken. Any ideas?
If I entered $-2 \times 5$, what would the output be? Why?
Hint 1:
I tried plugging is some more calculations. Also, the device reset or something before I started this, whatever that means. Anyways.
$24 + 3 = 6$
$-3 \times 1 = 9$
$-4 \div 2 = 2$
[No reset occurs between the main sequence and $-2 \times 5$ being entered in the puzzle itself]
Hint 2:
Since the device is a computer, I added the computer-science tag.
[You don't need to know any computer stuff to work this out - but this might help guide you in the right direction.]
Answer
You are using an unusual real-world device called
an RPN calculator.
Calculations:
The stack starts off containing 32 due to whatever you did before the puzzle, and the top-of-stack value is displayed on-screen (oddly the screen seems to show you the bottom of the stack not the top; I'm not sure about that bit). I have omitted some of the intermediate states. Stack contents are shown as
[bottom ... top].6 + [38]3 2 - [38 1]- [37]5 4 * [37 20]- [17]3 4 * [17 12]- [5]9 [5 9]
The = key seems to allow you to enter two consecutive numbers without them getting concatenated into one, so "3 2" would give you ones values on the stack:[32], but "3 = 2" puts two values on:[3 2]. Or maybe "=" causes the display of the bottom-of-stack rather than the top-of-stack that I'd expect from an RPN calculator.
And for the question:
- [-4]2 * [-8]5 [-8 5]
so "-8" would be displayed on-screen unless the "=" key is what causes the bottom-of-stack to be displayed, in which case the screen would show "5", because this final calculation doesn't have an "=".
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