2016 AMC 8 Problem 20

Attempt Problem 20 of the 2016 AMC 8 below, then check your answer against the video solution and professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AMC 8 solutions, or check the answer key.

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

20.

The least common multiple of aa and bb is 12,12, and the least common multiple of bb and cc is 15.15. What is the least possible value of the least common multiple of aa and c?c?
20 20
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60 60
120 120
180 180
Answer: A
Concepts:least common multiple
Small hint:
Since bb appears in both least common multiples, start with possible values of bb.
Big hint:
Try to make bb absorb the common factor 33 from 1212 and 1515.
Video solution:
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Written solution:
We know that bb has to divide both 1212 and 15,15, so it must equal either 11 or 3.3.
If b=1,b = 1, then a=12a = 12 and c=15,c = 15, making their least common multiple 60.60. If b=3,b = 3, then we may take a=4a = 4 and c=5.c = 5. The least common multiple in this scenario is 20.20.
Thus, A is the correct answer.
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