Using Sum in Report of Access 97

Dear All Expert;
  I have a problem to ask you...Now I am developing application about Tax.
It is Tax from every product and many group of product.
Example :
Can I design Report to find Average of Tax of each group of product ?
Structure :
Group A Have product 1,2,3,4
Group B Have Product 5,6,7,8

Beside Tax from each product , Can I find average Tax from Tax of each product
group ?
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When I do myself., I create Tax of each product group as Calculated Field in
Product Group Footer. But I can not take Tax of each product group from Product
Group Footer to find Average Tax of all group..( Average Tax of GroupA and
GroupB )

Please answer me...

Thank in advance