There are six boxes of goods in the store, weighing 15, 16, 18, 19, 20, and 31 kilograms respectively. Two customers bought five of them. It is known that the weight of the goods bought by one customer is twice that of the other customer. Then, What is the weight of a box of goods left in the store in kilograms?
商店里有六箱货物，分别重15、16、18、19、20、31千克，两个顾客买走了其中五箱，已知一个顾客买的货物重量是另一个顾客的2倍，那么，商店剩下的一箱货物重量是多少千克．
<result>20</result>