Sum of averages

[epjklomp]

Hello,

How can I create a measure, where the yearly outcome is the sum of averages, and the monthly outcome is not the sum of average, but the average itself....

For example:

date, country, working days per month, shipments, is_working_day 01-01-2008, NL, 22, 100, N 01-01-2008, BE, 20, 200, N 02-01-2008, NL, 22, 100, Y 02-01-2008, BE, 20, 200, Y 01-02-2008, NL, 18, 200, Y 01-02-2008, BE, 16, 200, Y

The measure "working days" uses as rollup an average and has the following outcome:
Jan = 21 - correct
Feb = 17 - correct
Total = 19 (averages of all working days per month) - wrong

Anybody knows how I get a Total of 38 in stead of 19?