2010 AMC 12B Problem 25

Attempt Problem 25 of the 2010 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 2010 AMC 12B solutions, or check the answer key.

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25.

For every integer n2,n\ge2, let pow(n)\operatorname{pow}(n) be the largest power of the largest prime that divides n.n. For example, pow(144)=pow(2432)=32.\operatorname{pow}(144)=\operatorname{pow}(2^4\cdot3^2)=3^2. What is the largest integer mm such that 2010m2010^m divides n=25300pow(n)?\prod_{n=2}^{5300}\operatorname{pow}(n)?

7474

7575

7676

7777

7878

Answer: D
Concepts:prime factorizationprimecounting integers in a range
Difficulty rating: 2640
Small Hint:

2010=23567;2010=2\cdot3\cdot5\cdot67; find the exponent of each prime in the product and take the minimum

Big Hint:

pow(n)\operatorname{pow}(n) contributes a prime pp only when pp is the largest prime factor of nn

Solution:

Since 2010=23567,2010=2\cdot3\cdot5\cdot67, write the product as 2A3B5C67D2^A3^B5^C67^D times a factor coprime to all four primes; then m=min(A,B,C,D).m=\min(A,B,C,D).

Prime 2:2: pow(n)\operatorname{pow}(n) is a power of 22 only when n=2k.n=2^k. Since 212=4096<5300<213,2^{12}=4096\lt5300\lt2^{13}, the values k=1,,12k=1,\ldots,12 contribute A=1+2++12=78.A=1+2+\cdots+12=78.

Prime 67:67: pow(n)=67\operatorname{pow}(n)=67 when 6767 is the largest prime factor, i.e. n=67jn=67j with 1j791\le j\le79 and every prime factor of jj at most 67;67; excluding j=67,71,73,79j=67, 71, 73, 79 leaves 7575 values. The one nn with pow(n)=672\operatorname{pow}(n)=67^2 is n=672<5300,n=67^2\lt5300, adding 2.2. So D=75+2=77.D=75+2=77.

Prime 3:3: For n=2a3bn=2^a3^b with b=1,2,3,4,b=1,2,3,4, the numbers of allowable exponents aa are 11,10,8,7,11,10,8,7, respectively. These terms alone contribute B11+210+38+47=83. \begin{aligned} B&\ge11+2\cdot10 \\ &\quad+3\cdot8+4\cdot7=83. \end{aligned}

Prime 5:5: Write n=2a3c5b.n=2^a3^c5^b. For b=1,b=1, the counts over c=0,,6c=0,\ldots,6 total 11+9+7+6+4+3+1=41.11+9+7+6+4+3+1=41. For b=2,b=2, the counts total 8+7+5+3+2=25,8+7+5+3+2=25, each contributing two factors of 5.5. Hence C41+225=91.C\ge41+2\cdot25=91.

Therefore m=min(78,B,C,77)=77.m=\min(78,B,C,77)=77.

Thus, the correct answer is D.

Problem 24#24
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