Tuesday, 15 December 2015

combinatorics - How many five digit numbers formed from digits 1,2,3,4,5 (used exactly once) are divisible by 12?




How many five digit numbers formed from digits 1,2,3,4,5 (used exactly once) are divisible by 12?





My answer is 24 but I doubt if it's right or not.



Sum of all the digits is 15, so all the numbers are divisible by 3. Also there are 24 numbers divisible by 4. I have found this by




  • Fixing 4 at units place , so I must place 2 at tens place and number divisible by 4 is 3!=6

  • Fixing 2 at units place, so I have 1,3 or 5 at tens place and number divisible by 4 is 3!×3=18




Since 12=3×4 and all numbers are divisible by 3 so numbers divisible by 12 is 24.



Is the reason valid?


Answer



Your decomposition of the problem is valid, and only works because those two divisors are co-prime (there is no number bigger than 1 dividing both divisors). This means that if a number is divisible by 3 and 4 it is automatically divisible by 12, and you can check each condition independently – which you did.


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