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Turn on your Macro Recorder, and record yourself setting up a Filter to only show numbers greater than zero (your positive numbers).
Then, add this following code that will delete all records that are not hidden, and remove the filter:
Code:
' Deletes all unhidden rows except for the header (first row only)
Application.DisplayAlerts = False
ActiveSheet.UsedRange.Offset(1, 0).Resize(ActiveSheet.UsedRange.Rows.Count - 1).Rows.Delete
Application.DisplayAlerts = True
' Remove filters
ActiveSheet.AutoFilterMode = False