A probability problem by xi guan

what is the coefficient of x 4 x^{4} in i = O 20 \sum_{i=O}^{20} ( 1 + 3 X ) i (1+3X)^{i}

27× 21 C 6 21C6 21 C 4 21C4 243× 21 C 7 21C7 81× 21 C 5 21C5

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