9x
The digits in each result add up to 9. For example: 72 (7 + 2 = 9)
The number in the 10s column progresses by 1.
The number in the 1s column decreases by 1.
And then there is the 9x table hand trick! A truly wonderful lesson.
https://youtu.be/jEIeFV4oMp4
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