What is Reddit's opinion of Casio FX-991ES Plus Scientific Calculator Fx 991 Es - New & Sealed ship to world wide?
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Don't forget how many 9 volt batteries those things would eat between 1974 and now. :D Even if you've got that RadioShack free battery card, old school TTL logic calculators were kind of a pain when it came to batteries.

Now once the 80s hit, and every calculator out there, just about, had a solar cell, you were more or less home free. The main limiting factor on those was how much abuse they could stand up to, and how easily you could end up losing them.

https://www.worthpoint.com/worthopedia/vintage-casio-scientific-calculator-219254008

https://casio.ledudu.com/images/calculs/casio/machines/zoom/FX115V300v.jpg

And its modernized cousin with quite a lot more creature comforts.

https://www.amazon.com/Casio-FX-991ES-Scientific-Calculator-991/dp/B00925QKBM/

https://www.amazon.com/Casio-FX-991EX-Engineering-Scientific-Calculator/dp/B00ZZ93346/