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2016 preRMO p7, coefficient of a^5b^5c^5d^6 in (bcd + acd + abd + abc)^7

Source:

August 9, 2019
combinatoricscoefficientalgebra

Problem Statement

Find the coefficient of a5b5c5d6a^5b^5c^5d^6 in the expansion of the following expression (bcd+acd+abd+abc)7(bcd +acd +abd +abc)^7