Once again, these are advanced strategies for students who know their number facts securely, and are ready to move on to more advanced topics.
The numbers 3 and 9 have an unusual, perhaps unique characteristic: for each of their multiples, the sum of the digits is also a multiple of 3 or 9. For example:
This feature of multiples of 3 and 9 can be used easily to test any whole number for divisibility by 3 or 9.
N.B. Watch the last few seconds of the video for an inadvertent error. I was embarrassed to see later that I made a mistake in checking 3-digit numbers for divisibility by 9. I hope no-one is too upset.