I have this problem with totals

In a report with a kind of structure like this :

group 1
group 2
month1 month2 month3 month4 . . . . . month12

weight
Value
----------------
group2 footer

tot weight
tot value
-------------
group 1 footer

well, i can have the totals weight and value using the "calculate" function of reports, but i dont' know how to get the middle price sum(weight) / sum(value)

How can i do ?