[rules-users] Help writing DRL

I have following condition buying 3 pen gives 1 free pencil buying 5 pen gives 1 free rubber buying 10 pen gives 1 free chocolate so if some one buys 19 pen then his free items will be 1 chocolate due to 10 pen 1 rubber due to remaining (19-10) pen 1 pencil due to remaining (9-5) pen I have objects of sales_details where trnqty is the quantity purchased how to calculate the free items using drl