HP 35s Scientific Calculator - Manual (Page 116)

6-14 Entering and Evaluating Equations
To change the number, type the new number and press  . This new
number writes over the old value in the X–register. You can enter a number as
a fraction if you want. If you need to calculate a number, use normal
keyboard calculations, then press
 . For example, you can press 2
 5  in RPN mode, or press 2  5  in ALG
mode. Before pressing  , the expression will display in line 2, and
after pressing  , the result of the expression will display in line 2.
To cancel the prompt, press  . The current value for the variable remains in
the X–register and displays in right-side of the line two. If you press 
during digit entry, it clears the number to zero. Press  again to cancel the
prompt.
To display digits hidden by the prompt, press   .
In RPN mode,each prompt puts the variable value in the X–register and disables
stack lift. If you type a number at the prompt, it replaces the value in the X–register.
When you press
 , stack lift is enabled, so the value is saved on the stack.
The Syntax of Equations
Equations follow certain conventions that determine how they're evaluated:
How operators interact.
What functions are valid in equations.
How equations are checked for syntax errors.
Operator Precedence
Operators in an equation are processed in a certain order that makes the
evaluation logical and predictable: