Explanation

70, 71, 76, 70, 81, 86, 70, 91, 96, 70, 101, 106, 70,------

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First reason:

In this series, 5 is added to the previous number; the number 70 is inserted as every third number.

Second reason:

Number 70 is used in this series as every third number.  

you will carry on this series by adding 5 in previous number