Other than advanced, graphing calculators that take full blown formulas as input, ordinary pocket calculators are not infix.

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

It's really

  $ echo 2 | plus 3 | times 4 | divide 10 | ...

I remember doing math drills like this in elementary school. The teacher startws with a number and then calls out operations:

"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.