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So my son is starting to show an interest in mathematics, and being the awesome dad that I am, I decided to buy him a basic calculator. Being the cheapskate that I am, I ordered the cheapest one I could find, which happened to be from NoSlaves™ Calculator Co. (When only the cheapest calculator will do, make sure you buy from NoSlaves™, where we lovingly craft our calculators to almost adequate levels of quality, and totally don't use any slave labour... at all.)

Unfortunately (and surprisingly), when it arrived, there seemed to be something wrong with it... I plugged in a few quick test calculations:

1 + 1 = 1
4 + 3 = 9
8 - 3 + 5 = 6
24 + 51 = 84
50 - 74 + 39 = 98

Now, I like to complain as much as the next guy, so I went hunting through the instruction manual for the company's phone number. However, on the warranty and support information page, I found the following, scrawled in a strange, dark reddish, congealed ink of some sort:

2 - 3 + 4 = ?
80 - 17 = ?
4 - 1 + 2 - _ = ?

Curious, I entered in the calucluations... I didn't know what the underscore meant, so just ignored it, but regardless, the results were even more broken than my initial test calculations. By this stage I was bored, so I shrugged it off and went to bed.

Three weeks later, being the brilliant detective I am, I suddenly realised that the hand written message carried a cleverly concealed secret message. I immediately phoned the police department and was hailed as a hero.

What was wrong with the calculator? And, what was the mysterious hidden message?

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    $\begingroup$ strange, dark-reddish congealed ink...? You don't mean... blood? O_O $\endgroup$ – Joe Z. Jul 2 '15 at 4:02
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    $\begingroup$ "Totally don't use any slave labor at all." Help, I'm stuck in a calculator factory. $\endgroup$ – DonielF Jul 3 '17 at 3:30
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The problem with the calculator is:

It adds/subtracts the segments of the display, not the numbers.
So, 4 + 3 is indeed equal to 9

The hidden message, then, is:

HELP, all in capitals. The underscore in the last sum is the bottom line segment on the display.

Example for the first sum : Example for the first sum
http://www.uize.com/examples/seven-segment-display.html
Since it's clearly been written in blood, something foul is afoot at the calculator factory

Sorry, don't have access to much by way of image-editing stuff atm, so can't do the pretty pictures.

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    $\begingroup$ You don't think that message was written by a slave, do you? O_o $\endgroup$ – Kevin May 23 '15 at 2:18

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