A certain project requires 24 days for A alone, 36 days for B, and 60 days for C; it is known that all three teams have worked for an exact number of days and completed the task in no more than 18 days, so how many days does A work at least? ．
某项工程，单独做甲需妻24天，乙需要36天，丙需要60天；已知三个队伍都恰好干了整数天，且不超过18天完成了任务，那么甲至少干了多少天．
<result>5</result>