Maths Olympiad Prep

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, 2015

Number theory Difficulty 4.0 AMC 10/12 Find the answer Japan

How many non-square numbers are there among the positive factors of the number 60006000?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

[34]
Since 6000=2431536000 = 2^4 3^1 5^3, positive factors of 60006000 can be written as 2a3b5c2^a 3^b 5^c where a=0,1,2,3,4a = 0, 1, 2, 3, 4; b=0,1b = 0, 1; c=0,1,2,3c = 0, 1, 2, 3. Therefore, there are altogether 524=405 \cdot 2 \cdot 4 = 40 positive factors of 60006000. Among them, square numbers arise only when all of a,b,ca, b, c are even, and this happens only 312=63 \cdot 1 \cdot 2 = 6 times. Therefore, there are 406=3440 - 6 = 34 non-square factors of 60006000.

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