2016 AMC 12A Problem 22

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22.

How many ordered triples (x,y,z)(x,y,z) of positive integers satisfy lcm(x,y)=72,\text{lcm}(x,y)=72, lcm(x,z)=600,\text{lcm}(x,z)=600, and lcm(y,z)=900?\text{lcm}(y,z)=900?

1515

1616

2424

2727

6464

Answer: A
Concepts:least common multipleprime factorizationcasework
Difficulty rating: 2160
Solution:

Because lcm(x,y)=2332\text{lcm}(x,y)=2^3\cdot3^2 and lcm(x,z)=23352,\text{lcm}(x,z)=2^3\cdot3\cdot5^2, the factor 525^2 divides zz while neither xx nor yy is divisible by 5.5. Also 323^2 divides y,y, while neither xx nor zz is divisible by 32,3^2, and xx must have the factor 23.2^3.

Writing x=233j,x=2^3\cdot3^{\,j}, y=2k32,y=2^{\,k}\cdot3^2, and z=2m3n52,z=2^{\,m}\cdot3^{\,n}\cdot5^2, the lcm conditions require max(j,n)=1\max(j,n)=1 and max(k,m)=2.\max(k,m)=2. The first pair can be (1,0),(0,1),(1,1),(1,0),(0,1),(1,1), and the second can be (2,0),(2,1),(2,2),(0,2),(1,2).(2,0),(2,1),(2,2),(0,2),(1,2). Thus there are 35=153\cdot5=15 ordered triples.

Thus, the correct answer is A.

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