请教一道gwd的数学题，谢谢啦~

Theater M has 25 rows with 27 seats in each row. How many of the seats were occupied during a certain show?
(1)   During the show, there was an average (arithmetic mean) of 10 unoccupied seats per row for the front 20 rows.
(2)   During the show, there was an average (arithmetic mean) of 20 unoccupied seats per row for the back 15 rows.
 
  Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient. 
 
  Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient. 
 
  BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient. 
 
  EACH statement ALONE is sufficient. 
 
  Statements (1) and (2) TOGETHER are NOT sufficient. 

为啥答案是最后一个呢？ 

我不是NN

不过我认为

条件1 并没有给出后7排的座位情况

条件2 没有给出前12排的座位情况

条件1 和 2 放在一起，只知道前20排平均每排有10个空座，后15排平均每排有20个空座，空座分布不均，不能求出条件1和条件2重合的8排位子有多少个空位

所以答案选E