Average ->
Arg Input : Array | List 
    Price : < Arg > : < Float : +- > 
    Amount : < Arg > : < Integer >

Total_price, Total_amount = 0 ..

Do Total_price += (Price X) * (Amount X),
   Total_amount += Amount X
With X from Input

If Total_amount then
 Total_price / Total_amount
Else
 0
/
