There are six barrels of beer in the store, and their weights are 15, 16, 18, 19, 20, and 31 kilograms, respectively. A total of five barrels were sold in two days, and the weight of beer sold on the first day was twice that of the second day. So how many kilograms of beer are not sold in the store?
商店里有六桶啤酒，它们的重量分别是15，16，18，19，20，31千克．两天共卖出五桶，其中第一天卖出的啤酒重量是第二天的两倍．那么商店里没有卖出的啤酒是多少千克．
<result>20</result>