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.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

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
Small Hint:

A product is determined by the exponents of 2,3,5,2,3,5, so count the distinct exponent triples

Big Hint:

After fixing that exactly kk dice show 5,5, count the possible exponent pairs of 22 and 33 from the other m=nkm=n-k dice

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.

Problem 22#22
Full Exam

Problem 23 in Other Years

1950 AMC 12 · 1951 AMC 12 · 1952 AMC 12 · 1953 AMC 12 · 1954 AMC 12 · 1955 AMC 12 · 1956 AMC 12 · 1957 AMC 12 · 1958 AMC 12 · 1959 AMC 12 · 1960 AMC 12 · 1961 AMC 12 · 1962 AMC 12 · 1963 AMC 12 · 1964 AMC 12 · 1965 AMC 12 · 1966 AMC 12 · 1967 AMC 12 · 1968 AMC 12 · 1969 AMC 12 · 1970 AMC 12 · 1971 AMC 12 · 1972 AMC 12 · 1973 AMC 12 · 1974 AMC 12 · 1975 AMC 12 · 1976 AMC 12 · 1977 AMC 12 · 1978 AMC 12 · 1979 AMC 12 · 1980 AMC 12 · 1981 AMC 12 · 1982 AMC 12 · 1983 AMC 12 · 1984 AMC 12 · 1985 AMC 12 · 1986 AMC 12 · 1987 AMC 12 · 1988 AMC 12 · 1989 AMC 12 · 1990 AMC 12 · 1991 AMC 12 · 1992 AMC 12 · 1993 AMC 12 · 1994 AMC 12 · 1995 AMC 12 · 1996 AMC 12 · 1997 AMC 12 · 1998 AMC 12 · 1999 AMC 12 · 2000 AMC 12 · 2001 AMC 12 · 2002 AMC 12A · 2002 AMC 12B · 2003 AMC 12A · 2003 AMC 12B · 2004 AMC 12A · 2004 AMC 12B · 2005 AMC 12A · 2005 AMC 12B · 2006 AMC 12A · 2006 AMC 12B · 2007 AMC 12A · 2007 AMC 12B · 2008 AMC 12A · 2008 AMC 12B · 2009 AMC 12A · 2009 AMC 12B · 2010 AMC 12A · 2010 AMC 12B · 2011 AMC 12A · 2011 AMC 12B · 2012 AMC 12A · 2012 AMC 12B · 2013 AMC 12A · 2013 AMC 12B · 2014 AMC 12A · 2014 AMC 12B · 2015 AMC 12A · 2015 AMC 12B · 2016 AMC 12A · 2016 AMC 12B · 2017 AMC 12A · 2017 AMC 12B · 2018 AMC 12A · 2018 AMC 12B · 2019 AMC 12A · 2019 AMC 12B · 2020 AMC 12A · 2020 AMC 12B · 2021 AMC 12A Spring · 2021 AMC 12B Spring · 2021 AMC 12A Fall · 2021 AMC 12B Fall · 2022 AMC 12A · 2022 AMC 12B · 2023 AMC 12A · 2024 AMC 12A · 2024 AMC 12B · 2025 AMC 12A · 2025 AMC 12B