/// Takes a list of (quantity/amount and price/rate) as decimals, and gives a weighted average
let weightedAvg mylist =
    let weightedValue = mylist |> List.sumBy(fun (isum,irate) -> isum*irate)
    let weightedSum = mylist |> List.sumBy(fun (isum,irate) -> isum)
    match weightedSum with
    | 0m -> 0m
    | x -> weightedValue / x