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Joined: Jan 2005
It sounds like the carry flag is set. If it were a 16C I could tell you exactly how to clear it, but I haven't used a 12C. Judging from a picture of the keyboard, I'd guess it's the blue shift key plus either the PV or PMT key. (It looks to me like the PV key is the right one, but I can't be sure.) If you have a manual, there ought to be a section in it about setting and clearing flags.
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Joined: Jan 1970
Hello Eric,
the "C" indicates "simple interest for fractional compounding periods". The HP-12C uses the exact mathematical method by default. Note that calculations with fractional periods are meaningless in most cases. Press STO EEX to switch back to default mode. I hope this helps so far.
Greetings from Germany Achim