free goods

hi all

i have a query in alloting free goods

if i want to configure freegoods for an item like this

for every 10 one is free

for every 80 9 are free

if customer purchases 680 the system should give

for 640 multiple of 80 i.e. 9x8 = 72 and for the balance 40 free goods wil be 4

total 76.

how to do it?

regards

rag