Encountered this question, with a solution, that I didn't understand...

How many solutions are there for the equation: $$ x_1+x_2+x_3+x_4+x_5+x_6=24 $$ Given that $$ x_1+x_2+x_3>x_4+x_5+x_6$$

There is a hint:

" A solution for the equation should be exactly one of these: $$ x_1+x_2+x_3>x_4+x_5+x_6 $$ $$ x_1+x_2+x_3<x_4+x_5+x_6 $$ $$ x_1+x_2+x_3=x_4+x_5+x_6 $$ "

The solution for the question says that if the requested number is $x$ , then because of symmetry: $ 2x + (D(3,12))^2 = D(6,24) $

  • Note: I don't know if it's a common symbol, but in our course $D(n,k) = \binom{n-1+k}{k}$

I understand that $D(6,24)$ is the number of solutions to the original equation, without the additional ineqaulity info.

I understand that $D(3,12)$ is the number of solutions for $ x_1+x_2+x_3=12 $

But how do I continue? I don't understand the other parts of the solution...


1 Answer 1


Addition is commutative and associative so there's nothing in the expression that distinguishes any $x_i$ from the other. So any fact about them should hold true under a permutation of the indices. In particular,

The number of solutions for which

$$x_1 + x_2 + x_3 > x_4 + x_5 + x_6$$

should equal the number of solutions for which

$$x_4 + x_5 + x_6 > x_1 + x_2 + x_3$$ (which can be re-written as: $x_1 + x_2 + x_3 < x_4 + x_5 + x_6$)

The reason why you need $D(3,12)^2$ is because when $x_1 + x_2 + x_3 = x_4 + x_5 + x_6$, we know that $x_1 + x_2 + x_3 = 12$ and $x_4 + x_5 + x_6 = 12$. Because we know how many solutions there are to each of these, we can multiply the number to get how many possible combinations there are for all $x_1, ..., x_6$ in this case.

  • $\begingroup$ and why $(D(3,12))^2 $? $\endgroup$
    – Zvika
    Sep 13, 2017 at 15:32
  • $\begingroup$ I'll edit the answer $\endgroup$
    – muzzlator
    Sep 13, 2017 at 15:32
  • $\begingroup$ great. And why can I be sure that there is such symmetry? shouldn't I prove it? $\endgroup$
    – Zvika
    Sep 13, 2017 at 15:33

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