public decimal CalculateInterest(IAccount account)
        {
            var amount = account.SumTransactions();

            if (amount <= 0)
                return 0;

            return amount * 0.001m;
        }
        public decimal CalculateInterest(IAccount account)
        {
            var amount = account.SumTransactions();

            if (amount <= 0)
                return 0;

            if (amount <= 1000)
                return amount * 0.001m;
            else
                return 1 + (amount - 1000) * 0.002m;
        }
        public decimal CalculateInterest(IAccount account)
        {
            var amount = account.SumTransactions();

            if (amount <= 0)
                return 0;

            DateTime tenDaysPrior = DateTime.Now.AddDays(-10);

            bool hasWithdrawalsInTenDaysPrior = account.Transactions.Any(t =>
                t.transactionType == Model.TransactionTypes.Withdrawal
                    && t.transactionDate >= tenDaysPrior);

            if (hasWithdrawalsInTenDaysPrior)
                return amount * 0.001m;

            return amount * 0.05m;
        }