The Social Security Administration has ordered an intensive check of 756 SSI payment recipients who are suspected of having above-standard incomes. Four clerical assistants have been assigned to this task. At the end of six days at 7 hours each, they have checked on 336 recipients. In order to speed up the investigation, two more assistants are assigned at this point. If they work at the same rate, the number of additional 7-hour days it will take to complete the job is, most nearly