The coefficient of x5 in the expansion of 1+x21+1+x22+...+1+x30 is

Question

The coefficient of

x^{5}

in the expansion of

{(1 + x)}^{21} + {(1 + x)}^{22} + . . . + {(1 + x)}^{30}

^{51} C_{5}

^{9} C_{5}

^{31} C_{6} -^{21} C_{6}

^{30} C_{5} +^{20} C_{5}

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