TITLE: Could the sum of an even number of distinct positive odd numbers be divisible by each of the odd numbers?
QUESTION [48 upvotes]: Could the sum of an even number of distinct odd numbers be divisible by each of the odd numbers ?
Let $k\geq 4$ be an even number. Can one find $k$ distinct positive odd numbers $x_1,\ldots,x_k$ such that each $x_i$ divides $S = \sum_{i=1}^k x_i$ ?
Is it possible at least for $k$ big enough ?

REPLY [89 votes]: Yes, it is possible.  Divide your sum by $S$ and you have 
$$1=\sum_i \frac {x_i}S$$
which is an Egyptian fraction expression of $1$ where all the denominators have the same number of factors of $2$.  This is known to be solvable with all denominators odd, but all known solutions have an odd number of terms.   A survey paper is here.  The section of interest is $9.5$.  One example is: 
$$1=\frac 13+\frac 15+\frac 17 + \frac 19+\frac 1{11}+\frac 1{15}+\frac 1{35}+\frac 1{45}+\frac 1{231}$$ 
where the denominators have least common multiple $3465$ so we can write:
$$3465=1155+693+495+385+315+231+99+77+15$$ 
with every term dividing the sum.  Now if we add $3465$ to each side we have a solution with an even number of terms:
$$6930=3465+1155+693+495+385+315+231+99+77+15$$
Any Egyptian fraction decomposition of $1$ into fractions with odd denominators yields a solution to your problem.  The sum will be twice the least common multiple of the denominators in the decomposition.  The paper shows that there is such a decomposition for all odd numbers of terms $9$ or above.  You can multiply any solution by any odd number to get another.  
What is happening is we are converting the Egyptian fraction decomposition of $1$ with all denominators odd into one that looks like $$1=\frac 12+\frac12\left(\text{all other terms}\right)$$