You can not put a value and a formula in a single cell.
For that we need to create a macro.
Copy the code below, right-click on the sheets tab and select View Code.
Paste the code in the big white field, which you can then close.
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("L4:L24")) Is Nothing Then Exit Sub
If Target.Value = 90 Then Target.Value = 100
If Target.Value = 80 Then Target.Value = 85
If Target.Value = 70 Then Target.Value = 60
Back at Excel try entering the specified values in the specified range to see if it works as you foresaw.