Terminated: unknown
This is one of a series of CASIO "Mini" calculators, the first one of which was introduced as a low price calculator in 1972.
So far I have not been able to find out the real purpose of the CNG key. What is does is subtract the previous result from the next number entered.
An example. First clear the calculator by pressing AC. The display now shows "0".
Now press 5 followed by CNG and the display shows "5" (zero was subtracted from 5).
Now press 2 followed by CNG and the display shows "-3" (5 was subtracted from 2).
Now press 3 followed by CNG and the display shows "6" (-3 was subtracted from 3).
If anyone can explain me the actual purpose of this function, please let me know. This page on casio-calculator.com (link validated 2025-04-16) shows an advert in German from around 1975 in which the CNG key is referred to as the "Geldrückgabeermittlungstaste" ("money return mediation key").
Multiplications can be repeated by pressing × twice. So, to repeatedly multiply a number by two, enter the number followed by × × 2 and pressing = repeatedly.
One would expect the same kind of behaviour with the ÷ key but its behaviour is unexpected. For instance, when entering 8 followed by ÷ ÷ 2, the first result is 0.25 and not 4. That is because when pressing ÷ twice, the operands are somehow swapped! So to repeatedly divide 8 by 2 type 2 followed by ÷ ÷ 8 and pressing = repeatedly.
The calculator runs on batteries as well as on a power adapter.