Quandaries and Queries


how do you test a number to see if it is divisible by 7 or not?
can you devise a test that works other than trial dividing? please and thank you :)



To see if an integer n is divisible by 7, take the last digit, multiply it by 2 and subtract the result from the number formed by removing the last digit from n. This number is divisible by 7 if and only if n is divisible by 7.

For example let n = 693

69 - 2 3 = 63

Since 63 is divisible by 7 so is 693.



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