Question 38712
) There are four letters and four envelopes. If the letters are placed in the 
envelopes at random, what is the probability that not more than one letter is placed in the correct envelope?

Probability that a letter is placed in the correct envelope = 1/4
Prob that "not more than one letter is correctly placed" means 
0 or 1 are correctly placed.
Prob (0 are correctly placed) = (3/4)^4=0.3164
Prob (1 is correctly placed) = 4C1 (1/4)(3/4)^3=0.421875

Sine there are mutually exclusive events you add the probabilities
to get 0.73828125

Cheers,
Stan H.