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Ending by 3 or 7 (Posted on 2016-08-01)

Any number ending by 3 or 7 must have at least one factor ending by 3 or 7.
Prove the above statement.

See The Solution Submitted by Ady TZIDON

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q.e.d.

Comment 1 of 1

Create a Mod 10 multiplication table.

The only way to get a 3 is 1*3 or 7*9

The only way to get a 7 is 1*7 or 3*9

Since all of these contain a 3 or a 7, we are proved.

Posted by Steve Herman on 2016-08-01 08:49:12

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