625 is a fairly common number on the GRE along with other powers of 5. Whenever I have a variable on a QC I like to ask myself can the number be Pos/neg, integer/nonint, or 0. Because \(x^4\) is an integer, I know that X is also an integer, and that it must be positive (therefore not 0) as well.
Therefore, I know X is positive 5. For the greatest prime factor, I don't ever need to multiply 36 and 5 together. I can just do the prime factorization of 36 (2233) and I have 5 so the greatest prime factor is 5 and both quantities are equal.
Remember if you can get away with not doing math on the quant section that's the way to go!
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