Many creditors of your firm offer early payment discounts. The accounts payable supervisor does not believe in paying early "as the bank overdraft rate of 8% pa is more than the average 2% offered for payment within 10 days from date of invoice". The supervisor stretches the accounts to 40 days from the last date of early payment discount. If average creditors terms are 30 days net, what is the minimum number of days from the net period that accounts must be stretched to make stretching a viable alternative?