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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