Array F# 2

        let swA = new System.Diagnostics.Stopwatch()
        let swB = new System.Diagnostics.Stopwatch()
        let arr = [| 1 .. 1800 |]

        swA.Start()
        let mutable sum1 = 0
        for i in 0 .. 1799 do
            sum1 <- sum1 + i * arr.[i]
        done
        swA.Stop()

        swB.Start()
        let fProdSum arr = arr |> Array.mapi (fun i j -> i * j) |> Array.sum
        let sum2 = fProdSum arr
        swB.Stop()

        printfn "Results: %d %d" sum1 sum2
        printfn "Time elapsed Mine: %A" swA.Elapsed
        printfn "Time elapsed His: %A" swB.Elapsed