Labeling Cells with Text but cell value is a number

Say that I need cell C9 to be shown to spreadsheet users as "TPT-2" but I need the value of the cell to be a numerical value so that I can use C9 in a formula.
How do I do this?

Thanks!

You can use custom number formatting.
You find this in the menu:
Format -> Cells -> Number -> Custom .

You can then use this custom numberformat : "TPT-"0

When you then enter 2 in the cell, the value in the cell will be 2 and can be used for calculations, but it will display as TPT-2

Thank you so much!