2013 AMC 10B Problem 22

Attempt Problem 22 of the 2013 AMC 10B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 10B solutions, or check the answer key.

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22.

The regular octagon ABCDEFGHABCDEFGH has its center at J.J. Each of the vertices and the center are to be associated with one of the digits 11 through 9,9, with each digit used once, in such a way that the sums of the numbers on the lines AJE,AJE, BJF,BJF, CJG,CJG, and DJHDJH are all equal. In how many ways can this be done?

384 384

576 576

1152 1152

1680 1680

3456 3456

Answer: C
Concepts:magic squaremodular arithmeticpairing and groupingmultiplication principle
Difficulty rating: 2060
Solution:

Let SS be defined as: S=A+J+E=B+J+F=C+J+G=D+J+H \begin{aligned} S &= A+J+E \\ &=B+J+F \\ &=C+J+G \\ &=D+J+H \end{aligned} 4S=A+B+C+D+E+F+G+H+4J \begin{aligned} 4S &= A+B+C+D+E \\ &\quad+F+G+H+4J \end{aligned} 4S=45+3J4S = 45+3J 45+3J0mod445+3J \equiv 0 \mod 4 3J3mod43J \equiv 3 \mod 4 J1mod4J \equiv 1 \mod 4

This means that J=1,5,9.J=1,5,9. From here, let's assume J=1.J=1. We will see that the other cases are similar enough to omit.

If J=1,J=1, then we know that the pairs of numbers that satisfy the equality above are: 2+9=3+8=4+7=5+62+9 = 3+8 = 4+7 = 5+6 There are 4!4! ways to distribute the pairs over the four groups, and then 242^4 ways for these groups to swap elements (i.e. 2+9    9+22+9\iff 9+2).

Now, if we look at the J=5J=5 and J=9J=9 cases, we see a similar pattern in the number of groupings and swaps. As such, we have: 34!24=11523\cdot 4! \cdot 2^4 = 1152 possibilities.

Thus, the correct answer is C .

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