SOLUTION: Your class has just completed a unit on divisibility rules. One of the students asks why divisibility by numbers other than 3 and 9 can not be tested by dividing the sum of the di

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Question 174108: Your class has just completed a unit on divisibility rules. One of the students asks why divisibility by numbers other than 3 and 9 can not be tested by dividing the sum of the digits by the tested number. How do you respond?
Answer by bob123132(33) About Me  (Show Source):
You can put this solution on YOUR website!
No. Because some numbers are factors of 3 AND 5 which would make them be counted twice. Take this for example up to 20:
3:3,6,9,12,15,18
5:5,10,15,20
If you used the students way it would equal 10 different numbers. But if you have noticed, 15 has both so right there, there are only 9 DIFFERENT numbers