pietropietro wrote:
I can get to 324 = 2^2 * 3^4 easily by decomposing the number, but not 289 because it's a prime number. So how am i supposed to know that 289 is 17 squared? How many number squares am I supposed to remember for the gmat?
I share your thoughts, the last part was harder for me too. But looking at 289 you'll see that, since it is not that easy to divide, you may try to use the alternatives and what you already have to solve this problem.
The answer is an integer, so "one" number squared will give you 289. You already have 18 (from sq. root 324), and the last digit of 289 is 9. So the root must be 13 or 17 (because the digits will give the 9 digit when multiplied). 18 + 13 is 31, so NO. 18 + 17 will give you 35, so answer D.