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If the number has a + sign then you add it, if it has a - sign, then you subtract it.
The scientific name for the number sign "#" is octothorpe.
The cent sign is put after the number, as opposed to the dollar sign, which is put before the number.
Octalthorp. Note that, in Britain the '#' sign is called 'hash' and isn't used as the number sign - we write 'No.' for number.
Think of it like this: