50g

I'd suggest downloading the HP-48 documentation.

Quick Start Guide
User's Guide
Advanced User's Reference Manual 

From Chapter 5 of the HP 48G Series User's Guide:

Where to Store Variables

When you evaluate a variable, the HP 48 searches the current directory for the name of the variable. If the name isn't there, the HP 48 searches up the path until it finds the variable or searches the HOME directory. Note that it will evaluate the first variable it finds with the designated name, which may or may not be the one you had in mind. This suggests several approaches to organizing your variables:

Put variables that you Want to access from any directory into the HOME directory

Put variables that you don't Want to be able to access from all directories into a
directory not on the current path.

You can use duplicate variable names, as long as they are not in the same directory.

To create the variable A in the PIPES directory, you need to place the following on the stack and press the STO key.  (Note that A is enclosed in single quotes)



Pressing the A softkey in the PIPES directory will place the formula on the stack.  Pressing the EVAL key will evaluate the formula using whatever contenet you have stored in the variable D that is in the HOME directory.