2016 AMC 12B Problem 24

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24.

There are exactly 77,00077{,}000 ordered quadruples (a,b,c,d)(a,b,c,d) such that gcd(a,b,c,d)=77\gcd(a,b,c,d)=77 and lcm(a,b,c,d)=n.\text{lcm}(a,b,c,d)=n. What is the smallest possible value of n?n?

13,86013{,}860

20,79020{,}790

21,56021{,}560

27,72027{,}720

41,58041{,}580

Answer: D
Concepts:greatest common divisorleast common multipleprime factorization
Difficulty rating: 2550
Solution:

Writing each entry as 7777 times a reduced value, we need gcd=1\gcd=1 and lcm=m=n/77.\text{lcm}=m=n/77. For each prime pp dividing mm with maximum exponent M,M, the number of valid exponent quadruples is (M+1)42M4(M+1)^4-2M^4 +(M1)4+(M-1)^4 =2(6M2+1).=2(6M^2+1). The total over all primes must equal 77,000=2353711.77{,}000=2^3\cdot5^3\cdot7\cdot11. Since 2(6M2+1)2(6M^2+1) equals 14,14, 50,50, and 110110 for M=1,2,3,M=1,2,3, and 1450110=77,000,14\cdot50\cdot110=77{,}000, the exponents 1,2,31,2,3 give one candidate factorization.

Every prime contributes exactly one factor of 2,2, so exactly three primes divide m.m. Their odd factors 6M2+16M^2+1 must divide 77,000/23=9625.77{,}000/2^3=9625. Checking the divisors gives M=1,2,3,8,M=1,2,3,8, with odd factors 7,25,55,385.7,25,55,385. The choice M=8M=8 leaves only 2525 for the product of the other two odd factors, but each is at least 7,7, so this is impossible. Therefore the maximum exponents are exactly 1,2,3.1,2,3. To minimize m=n/77,m=n/77, assign the largest exponent to the smallest prime: m=23325=360,m=2^3\cdot3^2\cdot5=360, so n=77360=27,720.n=77\cdot360=27{,}720.

Thus, the correct answer is D.

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