Monday, 20 July 2015

divisibility - How many 4-digit numbers with 3, 4, 6 and 7 are divisible by 44?





Consider all four-digit numbers where each of the digits 3, 4, 6 and 7 occurs exactly once. How many of these numbers are divisible by 44?







My attack:



There are 24 possible four digit numbers where 3, 4, 6 and 7 occur exactly once. I thought of writing them all down and checking divisibility, but isn't there a better way to do this?




Also, how do I check divisibility by 44 easily? I read on the internet there was a trick* to determine if a number is divisible by 11, but a number which is divisible by 11 doesn't have to be divisible by 44, does it?



*For example 3729, you write down (7+9)(3+2)=11, which is divisible by 11, so 3729 is divisible by 11.



I'm only looking for a hint.


Answer



For a number to be divisible by 4, the last 2 digits have to form a 2-digit number that is divisible by 4. This should simplify things a lot.



The trick for 11: you already know.




And if ABCD is divisible by both 4 and 11, it is divisible by 44.


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