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Number theory Difficulty 2.5 Junior Find the answer

What is the tens digit of the smallest positive integer that is divisible by each of 20, 16, and 2016?

A number or a short expression. Spacing and $ signs are ignored.

Solution

We note that 20=22520=2^{2} \cdot 5 and 16=2416=2^{4} and 2016=16126=253272016=16 \cdot 126=2^{5} \cdot 3^{2} \cdot 7. For an integer to be divisible by each of 2252^{2} \cdot 5, 242^{4}, and 253272^{5} \cdot 3^{2} \cdot 7, it must include at least 5 factors of 2, at least 2 factors of 3, at least 1 factor of 5, and at least 1 factor of 7. The smallest such positive integer is 25325171=100802^{5} \cdot 3^{2} \cdot 5^{1} \cdot 7^{1}=10080. The tens digit of this integer is 8.

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