ধারার ভেতর 7

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A new series is to be created after erasing some numbers from the series $1, 2, 3, 4, \dots , 399, 400$, in such a way so that the sum of any two numbers from the new series are not divisible by $7$. What is the maximum possible number of terms to be found in the new series?

Source: BdMO

Number Theory

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First Solve @wasimur

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