I could not understand this problem.

[defeated_soldier]

there are 6letters and 6 addressed envelopes .If one letter is assigned to one envelope randomly,then in how many ways can exactly 4 letters go into the right envelope ?


I could not understand this problem.

what it wants to mean by If one letter is assigned to one envelope randomly,
so this means......for each letter there is one envelope.


then in how many ways can exactly 4 letters go into the right envelope ?


I am now confused withis statement. what it wants to say ? is not 4 letters goes to 4 envelopes ? what does it mean when they say 'right' envelope ? everything is random.....is not it ?

 

[Denis]

Label the 6 envelopes 1 to 6 and the 6 letters 1 to 6,
with letter1 belonging to envelope1, letter2 to envelope2 and so on.

The 6 letters are mixed up: you pull one of them at random;
you then insert that letter (NOT looking at the number on it)
in an envelope: is the number on the envelope SAME as envelope number?

You continue until all 6 letters are in envelopes:
what is the probability that at least 4 letters were put in the correct envelopes?

Does that clarify it for you?