thank both i was playing with it and think i got it ..

=(SUM('July 01'!F6,'July 02'!F6,'July 03'!F6,'July 04'!F6,'July 05'!F6,'July
06'!F6,'July 07'!F6,'July 08'!F6,'July 09'!F6,'July 10'!F6,'July 11'!F6,'July
12'!F6,'July 13'!F6,'July 14'!F6,'July 15'!F6,'July 16'!F6,'July 17'!F6,'July
18'!F6,'July 19'!F6,'July 20'!F6,'July 21'!F6,'July 22'!F6,'July 23'!F6,'July
24'!F6,'July 25'!F6,'July 26'!F6,'July 27'!F6,'July 28'!F6,'July 29'!F6,'July
30'!F6)+SUM('July 31'!F6))/SUM(COUNT('July 01'!F6,'July 02'!F6,'July
03'!F6,'July 04'!F6,'July 05'!F6,'July 06'!F6,'July 07'!F6,'July 08'!F6,'July
09'!F6,'July 10'!F6,'July 11'!F6,'July 12'!F6,'July 13'!F6,'July 14'!F6,'July
15'!F6,'July 16'!F6,'July 17'!F6,'July 18'!F6,'July 19'!F6,'July 20'!F6,'July
21'!F6,'July 22'!F6,'July 23'!F6,'July 24'!F6,'July 25'!F6,'July 26'!F6,'July
27'!F6,'July 28'!F6,'July 29'!F6,'July 30'!F6)+COUNT('July 31'!F6))

Cheers,
Mayte