Question

The numbers $1, 2, 3, 4, 5$ are to be arranged in a circle. An arrangement is bad if it is not true that for every $n$ from $1$ to $15$ one can find a subset of the numbers that appear consecutively on the circle that sum to $n.$ Arrangements that differ only by a rotation or a reflection are considered the same. How many different bad arrangements are there?

$1$

$2$

$3$

$4$

$5$

Answer: B

Concepts:circular arrangements complementary counting casework

Difficulty rating: 2150

Solution:

Any single number covers sums $1$ through $5.$ If a consecutive block sums to $n,$ the remaining numbers form a consecutive block summing to $15 - n,$ so sums $10$ through $14$ are automatically covered as well. Thus an arrangement is bad only if it fails to produce $6$ or $7.$

If $6$ cannot be formed, rotate and reflect so the order is $1bc5e.$ The pair $\{b,c\}$ cannot be $\{2,3\}$ or $\{2,4\},$ so $e=2;$ avoiding the block $213$ then forces $14352.$ If $7$ cannot be formed, write the order as $2bc5e.$ Now $\{b,c\}$ cannot be $\{3,4\}$ or $\{1,4\},$ so $e=4,$ and avoiding $421$ forces $23154.$

These are the only two bad arrangements up to rotation and reflection.

Thus, the correct answer is B.

2014 AMC 12B Problem 18

Answer: B

Difficulty rating: 2150

Solution:

Problem 18 in Other Years