Hey guys,
I love that first question, and shovan85 I think you did a great job of explaining it! I may just chime in with some strategic advice if that's okay. Here's how I'd break it down:
To compare exponents, you'll need to find similar bases, so my goals will be:
1) Eliminate choices that just don't have a chance so that I minimize the work.
2) For anything that deserves consideration, find a common base to be able to compare.
3) Because it's asking for the greatest value, run it "tournament style" and only compare each remaining answer choice to the current "leader" for the sake of efficiency.
So...
A: 999^12 is close enough to 1000^12 for comparison that you can call it 1000^12 or (10^3)^12 = 10^36.
B: 10^30 is clearly less than 10^36, so 10^36 (A) still leads
C: 777^10 you could call (3/4 * 1000)^10. 1000^10 is the same as (10^3)^10 or 10^30, and since we know that the 3/4 portion will significantly reduce that (3/4^10 will get pretty small), A still leads.
D: -20^24 is going to be positive so you can ignore the negative. 20^24 = 2^24 * 10^24. This one gets interesting, I think, as 10^24 is obviously smaller than 10^36, our current leader, but 2^24 will be pretty big. Here it's helpful to find common, or at least comparable, bases. 2^24 is the same as (2^3)^8, and we'll use that because it's easier to compare 8 and 10 than it is to compare 2 and 10. 8^8 * 10^24 will clearly be less than 10^36, so D is eliminated.
E: sqrt 15 is the same as 15^1/2, so we can express this as (15^1/2)^40, or 15^20. This should pretty clearly be less than 10^36, but for additional review you might call it (3/2 * 10)^20, or 3/2 ^20 * 10^20. We already know that 2^24 wasn't enough to bump D above 10^36, so 3/2^20 obviously won't do it, and we can eliminate E as well.
The key on most exponent problems is to find common bases, and this one is a great example of that. Thanks for posting this one!
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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