What is the next number in this sequence 7 21 8 72 9.....I need to answer this question for a test and math has been a long time ago. What is the best way to relearn math sequences and probability and statistic.

I guess the answer is 729.

Series is 7 21 8 72 9

showing 7*3= 21

8*3^2=8*9=72

Following this series next number could be 9*9^2=9*81=729

another pattern could be

7*3=21

8*3^2=72

9*3^3=243

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