This is VBA territory, and requires a Worksheet_Change macro. This macro fires every time a cell on the worksheet is changed.
Open the VBA Editor. Double click on the sheet you are interested in and paste in the following code:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim sMiles As Single
If Target.Value = "Travel Mileage" Then
sMiles = InputBox("Please Enter Mileage", "Travel Mileage")
Target.Offset(0, 1) = 0.55 * sMiles
Target.Value = "Travel Mileage " & sMiles
End If
End Sub
You need to ensure the text exactly matches (capitalisation, etc), but if you're using drop downs this is easy.
Best regards, Rob.
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