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Khan Academy
Khan Academy: Divisibility Tests: Divisibility Tests for 2, 3, 4, 5, 6, 9, 10
Worked example of basic divisibility tests.
Khan Academy
Khan Academy: Divisibility Tests: The Why of the 3 Divisibility Rule
Why you can add the digits to see if something is divisible by 3.
Khan Academy
Khan Academy: Divisibility Tests: The Why of the 9 Divisibility Rule
Why you can test divisibility by 9 by just adding the digits.