[humptybumpty]

But what about:

4 -= 1 +=

4 -= 1 -=

I’m guessing 3 and 3?

[thih9]

My guess is different; these calculators did support negative numbers; I’d say:

4 –= 1 += gives: -3

4 –= 1 –= gives: -5

Manual for a similar model: https://www.oldcalculatormuseum.com/m-sharpqt8d.pdf