I have a doubt on the question solved by gregmat on the practice video of integers, primes, factors, multiples, fractions, decimals, percents and ratios. The question is: “if f(n) = n!, than what are the number of odd factors of f(10)”. I proceeded just as he does in the video, by doing the prime factorization and taking the product of the exponents +1 (each) of all the prime factor besides 2, which yields 30. However, I thought he was only including the positive factor, so the answer should’ve been 60. Why was it 30? in GRE questions like these, should I only consider the positive factors?
I think that’s a GMAT problem? GMAT problems only consider positive factors.
Even in GRE, you normally have to consider only positive factors (ETS would make this clear) - it’s more of a GregMAT thing to consider negative factors.
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Ah, ok. I also saw magoosh questions also including these negative factors and now I pay attention never to fall on this trick again. Thank you!