4 different letters and 4 addressed envelopes.

Question

Question is : In how many ways can the letters be put into the envelopes, so that at least one letter goes to the correct address ?

Answer is 15.
All i could think of is : 4 envelop choices for each letter, so 4*4 = 16.

A little fertilizer for my withered grey-matter ?

You have to subtract the number of derangements of four objects from the $4!$ ways of placing one letter in each envelope. — N. F. Taussig, Mar 06 '15 at 04:43

score 1 · Accepted Answer · answered Mar 06 '15 at 04:59

1

$$4!-!4=24-9=15$$

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answered Mar 06 '15 at 04:59

RE60K

What is !4? I have not seen this sign before. – Archisman Panigrahi Sep 14 '15 at 07:20

1 Answers1