问题求解答！！！！

加州海洋

有一题 DS 是这样的：
Robin drove from Townville to Villageton.  Upon arriving in Villageton, she immediately returned to Townville.  What was Robin's averge speed for the entire trip?
1) While driving from Townville to Villageton, Robin's averge speed was 40 miles per hour.  While driving from Villageton to townville, Robin's averge speed was 60 miles per hour.
2) The distance from Townville to Villageton is 120 miles.

我选择的是BOTH statements 1 and 2 TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient.


答案却是 --》 Statement 1 ALONE is sufficient to answer the question, but statement 2 alone is NOT sufficient.  
为什么呢？  #1 给的条件不足。  
请大家解释一下。  谢谢！！！

windsor1007

设路程为s你算一下，最后s是约掉的