My 2 cents:
For 1 letter to go in right envelope - probability = 1/4
For 2nd letter to go in wrong envelope - probability = 2/3
For 3rd letter to go in wrong envelope - probability = 1/2
For 4th letter to go in wrong envelope - probability = 1
Now, in the order this arrangement can be done:
C - Letter going into correct envelope
W - Letter going into wrong envelope
No#1. C - W - W - W
............in correct envelope: 4* (1/4) * (2/3) * (1/2) * 1 = 1/3
Thats the only explanation i was able to understand =(
I'm worried because if I don't learn the combinatrics formula way of doing it I might make a logical inconsistency while solving under a time limit.
Just wondering, is the above method correct? What if the correct is not the first one? Does it change the procedure?