Someone born in 1930s would have learned infix arithmetic in school just like somebody born in the 2010s.
Cannot work a remote, but can use RPN on an HP calculator? That sounds AI generated.
It's stateful and modal interfaces that are an extra step she never figured out, and some TV remotes require different modes for different functions, especially for different equipment. CEC has solved this on modern TVs, but it used to be common to have control the VCR/DVD player by selecting a mode, then control the TV by selecting another mode, and even without the other equipment, she could accidentally get the remote in the wrong mode.
She never got the hang of the states that are possible in a calculator with an interface using infix notation, so instead of using parentheses or order of operation, if she didn't have access to an RPN calculator, she'd write down results and enter them back as needed, to only have one operation in progress at a time, in the calculator.
They are prefix operations that take an implicit accumulator as an argument, resulting in something that superficially mimics a purely left-associative form of infix, with all operators having the same precedence.
For instance when we punch in 2 + 3 x 4 / 10 x 3 = = =, it means:
2 # prepare operand 2 in display
+ # move 2 into accumulator, prepare for addition
3 # prepare operand 3 in display
x # complete addition of acc 2 and operand 3, move 5 into acc and display
4 # prepare operand 4 in display
/ # complete multiplication of acc 5 and operand 4, move 20 into acc and dispay
10 # prepare operand 10 in display
x # complete division, moving 2 into acc and display
3 # prepare operand 3 in display
= # complete multiplication of acc 2 and 3, moving 6 into acc and display
= # complete multiplication of acc 6 and 3, moving 18 into acc and display
= # complete multiplication of acc 18 and 3, moving 54 into acc and display
$ echo 2 | plus 3 | times 4 | divide 10 | ...
"Start with 3; add 4; mutiply by 2; divide by 7; ....; ... write down the result."
I'm deeply skeptical that someone who had been through these drills would have trouble with a dollar store calculator.
My MIL can operate her phone and iPad but is constantly confused by the Roku and has to ask for help.