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Revision as of 17:26, 4 August 2006

Problem

Let the set $\mathcal{S} = \{8, 5, 1, 13, 34, 3, 21, 2\}.$ Susan makes a list as follows: for each two-element subset of $\mathcal{S},$ she writes on her list the greater of the set's two elements. Find the sum of the numbers on the list.

Solution

Each element of the set will appear in $7$ two-element subsets, once with each other number.

$34$ will be the greater number in $7$ subsets.

$21$ will be the greater number in $6$ subsets.

$13$ will be the greater number in $5$ subsets.

$8$ will be the greater number in $4$ subsets.

$5$ will be the greater number in $3$ subsets.

$3$ will be the greater number in $2$ subsets.

$2$ will be the greater number in $1$ subsets.

$1$ will be the greater number in $0$ subsets.

Therefore the desired sum is:

$\displaystyle 34\cdot7+21\cdot6+13\cdot5+8\cdot4+5\cdot3+3 \cdot2+2\cdot1+1\cdot0=484$

See also

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