Puzzle - 7 Continued....

There are two ways of finding out the colour of the pebble picked up by the girl. One by seeing the pebble actually picked up by her and the other by the seeing the pebble left in the bag. If the bag contains black pebble then she must have picked up the white one as the moneylender cannot say that he had put two black pebbles in the bag. In this case it would be good idea if the pebble left in the bag determines the colour of the pebble picked up the girl. She merely has to pick one, fumble so that it falls down on the path and as it is strewn with pebbles no one can make out the pebble picked up by her. Then the only way to find it out is by seeing the one left in the bag.

 2002 is here. Wish you a very happy New Year. I had stopped sending greeting cards by snail mail and few of my friends objected to it and I decided to send some (10 to be precise) by snail mail. I wrote down the names and the addresses on the envelopes and details on the cards. But then as usual lights went off. It was dark in the night and I could not put the cards inside the envelopes fearing that they may be send to the wrong person. I just wondered putting them in the envelopes at random and calculating probability of all of them being kept in the right envelopes or in wrong envelops.  It is difficult one so I asked one of my friend Prof. Ram Chandra Tiwari He tells me it is easy and there is formula to calculate it. The probability of all of them going right is 1/10! Or 1 divided by 362880; it is almost zero. The probability of putting all of them in wrong envelope is 0.36788 that is more than one third. But he asked me to tell him the probability of only one card being kept in the wrong envelope. I have not been able to do that. Can you help me?

He says that it does not require any formula. I am still in a fix.

By the way some of you might be wondering about Ram. He was my classfellow and was known as Sunil Dutt’s brother. He is at present with National Cancer Institute, Maryland, USA.