>>Thanks for your imput. Still the best explanation that I
>>read was Kevin Devins column, the link to which Alex posted
>>earlier.
>
>The name's Keith Devlin
>
>>7 chance of winning. See Kevin Devin's column at
>>http://www.maa.org/devin/devin_07_03.html
>
>The correct reference is
>
>http://www.maa.org/devlin/devlin_07_03.html

Actually his explanation is wrong. He writes:

1. The probability that the prize is behind door B or C (i.e., not behind door A) is 2/3.

2. The prize is not behind door C.

Combining these two pieces of information yields the conclusion that the probability that the prize is behind door B is 2/3. Endquote

In fact if the contestant chooses door A and Monty never opens
C when the car is behind A the chances on B are 100%