How many permutations of the word CAREFUL have EXACTLY TWO letters in the correct place?
correct place: C (first letter), A (second letter),...
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Two letters can be chosen out of 7 in ( 7 2 ) 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 ) [ e 5 ! ] = 2 1 × 4 4 = 9 2 4 .
Notes
A 'derangement' is an arrangement of objects so that no object is left in its original position.
! 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 = [ e n ! ]
where [ . ] is the nearest integer function. I think the formula, or at least its precursor, can be traced back to Euler.