Maths Olympiad Prep

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, 2010

Number theory Difficulty 5.0 AIME Prove it Estonia

Let aa, bb and cc be positive integers such that abab is divisible by 2c2c, bcbc is divisible by 3a3a and caca is divisible by 5b5b. Find the least possible value of abcabc.

Solution

Since abab is divisible by 2c2c and caca is divisible by 5b5b, abcaab \cdot ca must be divisible by 2c5b2c \cdot 5b, hence a2a^2 is divisible by 252 \cdot 5. Therefore a2a^2 is divisible by 22 and 55, hence aa is divisible by 22 and 55. Similarly bb is divisible by 22 and 33, and cc is divisible by 33 and 55. Consequently abcabc is divisible by 252335=9002 \cdot 5 \cdot 2 \cdot 3 \cdot 3 \cdot 5 = 900. On the other hand, a=10a = 10, b=6b = 6 and c=15c = 15 satisfy the conditions and abc=900abc = 900.

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