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Bunuel
The product of the largest two factors of a positive integer n is 16875. What is the difference between the largest positive integer that divides √n and the smallest odd factor greater than 1 of √n?

A. 2
B. 12
C. 13
D. 222
E. 223



Solution:

The largest factor of n is n, i.e., itself. The second largest factor is n/p where p is the smallest prime factor of n. For example, if the smallest prime factor of n is 2, then n/2 is the second largest factor of n. However, since the product of the largest two factors of n is 16,875, which is an odd number, we see that the largest factor of n (i.e., n itself) and its second largest factor must be odd also. To better get a sense what they should be, we should prime factorize 16,875:

16,875 = 25 x 675 = 5^2 x 25 x 27 = 5^2 x 5^2 x 3^3 = 5^4 x 3^3

We see that n must be 5^2 x 3^2 so that its largest factor is 5^2 x 3^2 (i.e., n itself), and its second largest factor is 5^2 x 3 (i.e., n/3). Therefore, √n = √(5^2 x 3^2) = 5 x 3 = 15.

The largest positive integer that divides √n is √n itself; so that is 15. The smallest odd factor of √n that is greater than 1 is 3. Therefore, their difference is 15 - 3 = 12.

Answer: B
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