服装厂里工人们正在制作一批校服，60名工人连续工作30天，每天工作9小时，正好能按期交货。在生产了5天之后，由于天气炎热，工厂领导临时决定每天下午上班时间由13：00推迟到14：00，其他时间不变。若该工厂想要确保能按期交货，则至少需要增加（ ）名效率相当的工人？（ ）

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