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 $\large{\textbf{a hint}}$.


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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