What is the rule? Challenging question for 8 year old students

A challenging problem was posted to Reddit HomeworkHelp and it was said to be given to gifted 8 year old students. Can you figure it out?

Reddit HomeworkHelp
https://www.reddit.com/r/HomeworkHelp/comments/prsfpa/3rd_grade_math_need_hel

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Видео What is the rule? Challenging question for 8 year old students канала MindYourDecisions