There are six boxes of oranges, apples, and pears. The weights of these six boxes are: 15, 16, 18, 19, 20, and 31 kilograms. The weight of apples is half that of pears, and there is only one box of oranges. How many kilograms does this box of oranges weigh?
桔子、苹果、梨共有六箱，这六箱水果的重量分别为：15、16、18、19、20、31千克，其中苹果的重量是梨的一半，桔子只有一箱．这箱桔子有多少千克．
<result>20</result>