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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