[thih9]

Thank you! This looks surprisingly intuitive, perhaps even comfortable, especially when muscle memory kicks in.

Then again…

I guess 4 += 1 += gives the answer 5.

What would be the result of 4 += 1 -= ?

Edit: the answer is “3”, elaborated on the linked page: http://www.vintagecalculators.com/html/calculator_keyboard_l...

[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