Wow!
Probability
Level
3
How many permutations of 1234567 have exactly one digit in the right place?
As an explicit example, 1726354 is an example of a permutation that have only one digit in the right place.
The answer is 1855.
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1 solution
There are 7 ways to pick the digit in which it will be in the right place. The 6 remaining digits should be completely deranged from each other.
So the answer is: =7×(derangement of 6) =7×265 =1855