Official explanation:
The question asks for the factor by which the value of a gem with a rating of p would have to be multiplied to equal the value of a gem with a rating of p – r. According to the rating system described by the question stem, the value of a gem with a rating of q – 1 is 5 times greater than the value of a gem with a rating of q. Subtracting 1 from the rating corresponds to a 5-fold increase in value. Following the pattern suggested by the question stem, the value of a gem with a q – 2 rating is greater than the value of a gem with a q rating by a factor of 25, or 5^2 . Similarly, the value of a gem with a q – 3 rating is greater than the value of a gem with a q rating by a factor of 125, or 5^3.
Apply this pattern to gems rated p – r and p. The easiest way to do this is to Plug In values of p and r. Let p = 5 and r = 2. In this case, the gems to be compared are rated ‘5 –2’ and ‘5’. Based on the pattern above, the value of the gem rated ‘5 – 2’ is greater than the value of the gem rated ‘5’ by a factor of 25. Substitute p = 5 and r = 2 into the answer choices, looking for the answer that equals 25.
Choice (A) is (5) 5 – (2) 5 , a number much greater than 25. Eliminate (A).
Choice (B) is (2) 5 , or 32. Eliminate (B).
Choice (C) is (5 – 2) 5 , or 243. Eliminate (C).
Choice (D) is 5^2 , or 25. Keep (D).
Choice (E) is 5^5×2 , or 5^10 , a number much greater than 25. The correct answer is (D).