Answer to Question #142113 in Discrete Mathematics for Promise Omiponle

Question #142113
How many solutions are there to the equation x1+x2+x3+x4+x5+x6= 25 where the xi(for i= 1,2,3,4,5,6) are non-negative integers?
1
Expert's answer
2020-11-24T08:09:38-0500

k-combination with repetitions, or multisubset of size kk  from a set SS  is given by a sequence of kk  not necessarily distinct elements of SS, where order is not taken into account: two sequences define the same multiset if one can be obtained from the other by permuting the terms. Associate an index to each element of SS  and think of the elements of SS  as types of objects, then we can let xi{\displaystyle x_{i}} denote the number of elements of type ii in a multisubset. The number of multisubsets of size kk  is then the number of nonnegative integer solutions of the Diophantine equation:


x1+x2+...+xn=kx_1+x_2+...+x_n=k


If SS  has nn  elements, the number of such kk-multisubsets is denoted by ((nk))({n \choose k}). This expression, nn  multichoose kk, can also be given in terms of binomial coefficients:


((nk))=(n+k1k)({n \choose k})={n+k-1\choose k}.


In our case, the number of nonnegative solutions of the equation


x1+x2+x3+x4+x5+x6=25x_1+x_2+x_3+x_4+x_5+x_6= 25


is equal to

((625))=(3025)=30!25!5!=30292827265432=142,506({6 \choose 25})={30\choose 25}=\frac{30!}{25!\cdot5!}=\frac{30\cdot 29\cdot 28\cdot 27\cdot 26}{5\cdot 4\cdot 3\cdot 2}=142,506




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