黑板上写着数9，11，13，15，17，19．每一次可以擦去其中任何两个数，再写上这两个数的和减1（例如，可以擦去11和19，再写上29）．经过几次之后，黑板上就会仅剩下一个数．试问，这个所剩下的数可能是多少？试找出所有可能的答案，并证明再无别的答案．
9+11+13+15+17+19-5=79；
答：经过5次之后，黑板上就会仅剩下一个数．这个所剩下的数就是79．
| 用方框中的句子补全对话，其中有两项多余。 | |
| |
| Bruce: Hi,Kate! How was your summer holiday'? Kate: _____1______ Bruce: Where did you go? Kate: I went to Yinchuan with my parents. Kruce: ______2_____ Kate: It's one of the most beautiful cities in the northwest of China. I like it very much Bruce: How long did you stay there? Kate: We stay there for five days._____3_____ Bruce: Which is your favourite place'? Kate: _____4_____When I was there, I lost myself in the beautiful screnery (景色 ) . Bruce: What interesting things did you do there? Kate: Lots of things, such as swimming, boating, fishing and riding camels. ____5_____ Brure: Oh , really? l can't wait to go there. |