There are six boxes of oranges, apples, and pears, and the weights are 15, 16, 18, 19, 20, and 31 kilograms respectively. Among them, 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?
橘子、苹果、梨共有六箱，重量分别是15、16、18、19、20、31千克，其中苹果重量是梨的一半，橘子只有一箱，这箱橘子重多少千克．
<result>20</result>