Quantitative Aptitude :: Permutation and Combination - Discussion

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Explanation:
If the digit at extreme right and extreme left are 5 and 6 respectively.
So, the 7 digit number is : 6 _ _ _ _ _ 5
Since repetition of digits are not allowed the second digit can be chosen from (0,1,2,3,4,7,8,9) 8 of the remaining digits.
5 spaces are left, hence 8P5=6720