query problem, group by key figure

Hi,

I have to build a report and need some help.

I have a cube with this data:

receipt | sales | card | counter

51 | 10 | 1001 | 1

52 | 30 | 1002 | 1

53 | 15 | 1003 | 1

54 | 20 | 1001 | 1

This is the report that I need:

We want to see how often our customers used their customer cards.

number of purchases | number of cards | sum of sales

The result of the sample data should be:

1 | 2 | 45 (cards 1002+1003)

2 | 1 | 30 (card 1001)

My problem is how to group by the number of purchases because it is not a characteristic but a key figure.

Any ideas?

Thanks,

Timo