问题如下图：
您好，请问答案为什么不是减少400啊？long方为什么不是1800个席位被closed out呢？
NO.PZ2020012001000017问题如下Suppose thon a particuly there are 3,000 tras in a futures contract. Of the buyers in those 3,000 tras, 1,800 were closing out positions an1,200 were taking new positions. Of the sellers in those 3,000 tras, 1,400 were closing out positions an1,600 were taking new positions. Whis the change in open interest ring the y? es it increase or crease? The open interest goes wn 200. This is because there are 1,200 new long positions an1,400 long positions are closeout (with short tras). Alternatively, there are 1,800 new short positions an1,600 are closeout (with long tras). 就是新签即买卖双方分别是1200和1600，双方交易1600—1200=400由于买卖是翻倍的合约400除以2=200这样理解对吗
NO.PZ2020012001000017问题如下Suppose thon a particuly there are 3,000 tras in a futures contract. Of the buyers in those 3,000 tras, 1,800 were closing out positions an1,200 were taking new positions. Of the sellers in those 3,000 tras, 1,400 were closing out positions an1,600 were taking new positions. Whis the change in open interest ring the y? es it increase or crease? The open interest goes wn 200. This is because there are 1,200 new long positions an1,400 long positions are closeout (with short tras). Alternatively, there are 1,800 new short positions an1,600 are closeout (with long tras). 该类题型如何着手，按照什么样的步骤解答，烦请写个步骤，综合看一下，谢谢。没有任何思路。
NO.PZ2020012001000017问题如下Suppose thon a particuly there are 3,000 tras in a futures contract. Of the buyers in those 3,000 tras, 1,800 were closing out positions an1,200 were taking new positions. Of the sellers in those 3,000 tras, 1,400 were closing out positions an1,600 were taking new positions. Whis the change in open interest ring the y? es it increase or crease? The open interest goes wn 200. This is because there are 1,200 new long positions an1,400 long positions are closeout (with short tras). Alternatively, there are 1,800 new short positions an1,600 are closeout (with long tras). 市场总量是剩下1600是吗
NO.PZ2020012001000017为什么要从sell方分析？从买方分析要怎么算？买方close out1800为什么不能减去新签的得加400份？
看了之前的两个解答，完全看不懂，能不能详细地讲解一下啊。比如什么叫close之前的空头？为什么之前的long方就是现在的short方？buyer方 taking 1200 new positions 是指跟seller方的1400 closing position 签合约吗？那还有200合约跟谁签的呢？
