The store buys 6 barrels at a time, and the weights are 15 kg, 16 kg, 18 kg, 19 kg, 20 kg, and 31 kg. 2 barrels are sold in the morning and 3 barrels are sold in the afternoon, and the amount of money sold in the afternoon is exactly twice that of the morning. How much does the remaining barrel weigh in kilograms?
商店一次进货6桶，重量分别为15千克、16千克、18千克、19千克、20千克、31千克．上午卖出去2桶，下午卖出去3桶，下午卖得的钱数正好是上午的2倍．剩下的一桶重多少千克．
<result>20</result>