Combi offer! #3

The number of non negative integral solutions of 3x + y + z = 24 is

225 108 117 None of these

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2 solutions

Tanya Gupta
May 30, 2014

There is a very basic method...put x=0,1,2.....8 and solve each equation using beggars or rather stars and bars method(Same method, different names)....the sum turns out to be 1+4+7....25, which is 117

Bill Bell
Sep 17, 2014

t = 3x can assume values in the set {0, 3, 6, 9, ... , 24}; y and z each in the set 0(1)24. Thus one can represent the possibilities for t by the generating function,

1 + x 3 + x 6 + x 9 + x 12 + x 15 + x 18 + x 21 + x 24 1+{x}^{3}+{x}^{6}+{x}^{9}+{x}^{12}+{x}^{15}+{x}^{18}+{x}^{21}+{x}^{24}

and those for x and y by

1 + x 1 + x 2 + x 3 + x 4 + x 5 + x 6 + x 7 + x 8 + x 9 + x 10 + x 11 + x 12 + x 13 + x 14 + x 15 + x 16 + x 17 + x 18 + x 19 + x 20 + x 21 + x 22 + x 23 + x 24 1+{x}^{1}+{x}^{2}+{x}^{3}+{x}^{4}+{x}^{5}+{x}^{6}+{x}^{7}+{x}^{8}+{x}^{9}+{x}^{10}+{x}^{11}+{x}^{12}+{x}^{13}+{x}^{14}+{x}^{15}+{x}^{16}+{x}^{17}+{x}^{18}+{x}^{19}+{x}^{20}+{x}^{21}+{x}^{22}+{x}^{23}+{x}^{24}

Multiply these three polynomials together and select the coefficient for x 24 {x}^{24} . I used Python sympy for this purpose.

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