Inspired by Paul Fournier

How many permutations of the word CAREFUL have EXACTLY TWO letters in the correct place?

correct place: C (first letter), A (second letter),...


Inspiration


The answer is 924.

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1 solution

Peter Macgregor
Apr 18, 2016

Two letters can be chosen out of 7 in ( 7 2 ) \begin{pmatrix}7\\2\end{pmatrix} ways.

Once two letters have been chosen and placed in their correct places the remaining 5 letters can be deranged in !5 ways.

Combining these two observations and using a very neat formula for the number of derangements of n objects gives

( 7 2 ) [ 5 ! e ] = 21 × 44 = 924 \begin{pmatrix}7\\2\end{pmatrix}\left[\frac{5!}{e}\right]=21\times44=\boxed{924} .

Notes

A 'derangement' is an arrangement of objects so that no object is left in its original position.

! n !n is the 'sub factorial of n' and is the symbol for the number of derangements of n.

The 'neat formula' states that the number of derangements of n objects is given by

! n = [ n ! e ] !n=\left[\frac{n!}{e}\right]

where [ . ] [.] is the nearest integer function. I think the formula, or at least its precursor, can be traced back to Euler.

Thank you for your solution (+1). I haven't realiced this wonderful formula for derangements works for small numbers too.

Guillermo Templado - 5 years, 1 month ago

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