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You can custom format cells in Microsoft Excel. In this example we wanted the letter "r" and the symbol "#" preceding (directly to the left) of each number that we entered.

In older version of Excel, click Format / Cells... / Number Tab / Custom, then enter the following into the "Type:" box:

For example, to place the letter "r" to the left of each number, enter "\r##0" in the "Type:" box and to place the character "#" to the left of each number, enter "\###0" in the "Type:" box.

Source: Excel Custom Number Formats