The number of 5 digit numbers in which no two consecutive digits are identical
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Solution
1st digit can be 1 to 9 = 9 ways 2nd digit can be 0 to 8 = 9 ways (we consumed 1 digit already in the 1st didgit) 3rd digit can be 0 to 7 = 8 ways 4th digit can be 0 to 6 = 7 ways 5th digit can be 0 to 5 = 6 ways