Find the number of ordered septuples of non-negative integers \(\left(x_1, x_2, x_3, x_4, x_5,x_6,x_7\right)\) such that \(0\le x_i\le 7\) for all \(i\) and \(5\) divides \(\displaystyle \sum_{i=1}^7 3^{x_i}\).

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