There are $6$ boxes each contains only oranges, apples, or pears. The boxes weigh $$15$$, $$16$$, $$18$$, $$19$$, $$20$$, and $$31$$ kg, respectively. If the total weight of apples is half of that of pears, and there is only one box of oranges, what is the weight of the box of oranges. 
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The total weight of all the boxes is $15+16+18+19+20+31=119$ kg. The weight of pears is twice that of apples, so excluding the box of orange, the weight of the remaining boxes should be divisible by $3$. Only removing $20$, the remaining weight is divisible by $3$. 