2023 AMC 12B Problem 23

Attempt Problem 23 of the 2023 AMC 12B 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 2023 AMC 12B solutions, or check the answer key.

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23.

When nn standard six-sided dice are rolled, the product of the numbers rolled can be any of 936936 possible values. What is n?n?

1111

66

88

1010

99

Answer: A
Concepts:prime factorizationsystematic listing
Difficulty rating: 2270
Solution:

Each die contributes an exponent vector in the primes 2,3,52,3,5 (face 1(0,0,0),1\to(0,0,0), 2(1,0,0),2\to(1,0,0), 3(0,1,0),3\to(0,1,0), 4(2,0,0),4\to(2,0,0), 5(0,0,1),5\to(0,0,1), 6(1,1,0)6\to(1,1,0)), and a product is determined by the sum of these vectors.

Fix the exponent of 55 by requiring exactly kk dice to show 5,5, and put m=nk.m=n-k. For an exponent bb of 3,3, where 0bm,0\le b\le m, the possible exponents aa of 22 are precisely 0,1,,2mb.0,1,\ldots,2m-b. If ab,a\le b, use aa faces showing 66 and bab-a showing 3.3. If a>b,a\gt b, use bb faces showing 66 and make the remaining aba-b factors of 22 with faces 44 and, if needed, one face 2.2. The bound a2mba\le 2m-b says this uses at most mm dice; fill unused dice with 11's. Thus the number of exponent pairs is b=0m(2mb+1)=(m+1)(3m+2)2. \begin{gathered} \sum_{b=0}^{m}(2m-b+1)\\ {}=\frac{(m+1)(3m+2)}2. \end{gathered}

Summing over k=0,1,,nk=0,1,\ldots,n, equivalently over m=0,1,,n,m=0,1,\ldots,n, the number of distinct products is m=0n(m+1)(3m+2)2=(n+1)2(n+2)2. \begin{gathered} \sum_{m=0}^{n}\frac{(m+1)(3m+2)}2\\ {}=\frac{(n+1)^2(n+2)}2. \end{gathered} For n=11n=11 this is 122132=936,\dfrac{12^2\cdot13}{2}=936, so n=11.n=11.

Thus, the correct answer is A.

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