let calcTax =
    let rec tax amt = function
    | (limit,rate)::rest when amt > limit ->
        rate * (amt - limit) + tax limit rest
    | _ :: rest -> tax amt rest
    | [] -> 0.0m

    let rates =
        [40230m, 0.45m;
         21240m, 0.33m;
         14070m, 0.30m;
          8660m, 0.23m;
          5070m, 0.14m;
             0m, 0.10m]

    fun amt -> tax amt rates