SQL calculation on filtered rescordset

[Atlas]

I have a form showing records from table A. On the same form there's a
subform showing data from table B, in a master/detail fashion.
Table B has some numeric fields: B.Income B.Outcome

Quite often a filter applies to recordset of table A. When that is the case,
I would like to make possible to calc sums on B.Income B.Outcome fields for
all the records in (filtered)->A->B.

How can I acheive that without reading all the A (filtered)-recordset
looping on all the childs in B?

Thanks