Help with an average measure excluding value

Hi

I was wondering if someone could help me? I have a score created through a measure, shown by day. This works fine. I can show the measure in a table of days and this gives me a rating per day and an average over a period of time.

But, if the value on a given day falls below a certain rating (20) I would like to exclude this from the average. The data containing the 20 does not exist as a column as it is a measure divised by day over a time period. This is my measure for the average:

Day Average = AVERAGEX(VALUES(DateKey[Date]),[Score])

I would like it to only considers score of above the level of 20 per day for the average, the [Score] is a measure and not a column.

Any help would be much appreciated.

Thanks

Ian