Two snails, A and B, crawl to the bottom of the well at the speeds of 20dm/day and 15dm/day respectively during the day, and slide down at the same speed at night. A climbs down in 5 days and B climbs down in 6 days. What is the depth of the well?
甲乙两只蜗牛往井底爬，白天速度分别为20dm/天，15dm/天，晚上向下滑的速度相同，甲5天爬下去，乙6天爬下去，井深多少．
<result>15</result>