A certain retailer's credit contract designates the rule of 78 for computing unearned interest and imposes a "prepayment penalty." In case of any payoff earlier than the due date, they will charge an additional 15% of the original finance charge. Find the least value of k (remaining payments after payoff) that would result in any net savings in the case of 24 payments originally scheduled. In order to result in any savings, the payoff must be at least monthly early