Delete Rows Multiple Criteria w/Numbers

Try this:

Sub DeleteRows()
    Dim iRow        As Long

    With ActiveSheet
        For iRow = .Cells(.Rows.Count, "B").End(xlUp).Row To 1 Step -1
            If .Cells(iRow, "B").Value2 > 900 / 24 And _
               InStr(1, .Cells(iRow, "C").Value, "unknown", vbTextCompare) Then
                .Rows(iRow).Delete
            End If
        Next iRow
    End With
End Sub