There are 6 bags of flour in the grain store, weighing 15, 16, 18, 19, 20, and 31 kilograms respectively. The cafeteria bought 5 bags of them twice. It is known that the weight of the first purchase is twice that of the second , What is the weight of the remaining bag in kilograms?
粮店里有6袋面粉，分别重15、16、18、19、20、31千克，食堂分两次买走了其中5袋，已知第一次买走得重量是第二次的两倍，剩下的一袋重量为多少千克．
<result>20</result>