[madongjun]

bill兄弟两和父母一起要在深夜过一桥。
条件：桥只能承受2人重量，过桥的人必须要用手电（只要并且只有一个手电）。弟弟过桥得3分钟，bill过桥得2分钟，父过桥得7分钟，母过桥得8分钟。
问：最短多久一家全部过桥？

[madongjun]

上述命题是阿肥大师提出来的，考考大家。
需要认真吸取各种信息，才能找到答案。

其实，桥牌也就是要通过叫牌打牌过程，捕捉各种信息，然后推理得到正确的运用，才能找到回家的路。

[lycier]

太难了，脑袋都算疼了，仍是一头雾水......
走个后门，偷偷告诉我吧

[dvd]

试试：3+7+8+2+2=22

[bgm]

dvd's attempt is a very natural one. One can easily see that the pattern will be

2 members forward -> 1 member backward -> 2 members forward -> 1 member backward -> 2 members forward

and to minimize the backward time, we will assign Bill as the man holding the torch and accompany all his family members forward. This will result in dvd's answer.

However, we shall note that there is clear wastage when Bill bring his parents forward in the above design. There will be less wastage when both parents go forward together. Namely,

Two Brothers forward (3) -> Bill backward (2) -> Parents forward (8) -> Brother backward (3) -> Two Brothers forward (3)

and the result will be only 19 mins. Note the order of backward can be exchanged in this design.

[madongjun]

bgm, on 2013-November-29, 06:15, said:

dvd's attempt is a very natural one. One can easily see that the pattern will be

2 members forward -> 1 member backward -> 2 members forward -> 1 member backward -> 2 members forward

and to minimize the backward time, we will assign Bill as the man holding the torch and accompany all his family members forward. This will result in dvd's answer.

However, we shall note that there is clear wastage when Bill bring his parents forward in the above design. There will be less wastage when both parents go forward together. Namely,

Two Brothers forward (3) -> Bill backward (2) -> Parents forward (8) -> Brother backward (3) -> Two Brothers forward (3)

and the result will be only 19 mins. Note the order of backward can be exchanged in this design.

Excellent!You are right!