Reports & Discoverer :: Averages Across The Columns

I have the following 6 columns in a report. And i need a 7th column which is a calculated field (average of the first 6 columns), the problem is some of the records have a '0' value in them so the aveage is not always (SUM)/6, If one of the fields have a 0 then the average is (SUM)/5, likewise if 2 fields have o's in them then the average will be (SUM)/4, see the sample data set.

A B C D E F G(Average)
83 83 33 0 0 100
83 83 33 0 0 100
0 67 67 100 17 92
83 83 100 67 50 100
83 83 100 83 50 100
67 0 83 100 17 100
83 83 83 67 67 100
83 83 83 50 50 90

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