Which one is better, the Casio 991EX or the 991ES Plus? - Quora
Casio FX-991EX and FX-300ES
Casio Scientific Calculator fx-991ex / fx-991ES PLUS / fx-991MS / fx-82ES + New | eBay
Eddie's Math and Calculator Blog: Comparison: Casio fx-991EX Classwiz Vs Sharp EL-W516T
Got ahold of a Casio fx-991ex and CG-50 to match my Classpad II. I'm really impressed with them but there is something about the fx-991ex... (see comments) : r/calculators
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Amazon.com : Casio fx-991ES Plus 2 Scientific Calculator with 417 Functions and Display, Natural : Office Products
Casio Launches New fx-991EX Scientific Calculator With Spreadsheet Function