In the spreadsheets, where an input value is required the cells are coloured light blue.  Output results appear in light yellow coloured cells. Advice or warnings may appear in the red coloured cells.


Before any valid inputs have been typed into the blue cells, the output cells may contain a zero or some other statement, such as #DIV/0!.  These will disappear as the necessary inputs are made.  Likewise, graphs will only be drawn correctly after all the necessary input values have been made. 


Each spreadsheet has been protected, although not with a password, such that inadvertent inputs that might corrupt a calculating cell are avoided.