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First: 15x15=225. Then, you can see that 16x16 will not be enough.. Just to make sure. 17x17=289! The other squad we can guest that it is 18! _________________

this ques is very eazy but difficult if don't know you perfect squares. I was able to get the first part very quick but I used calculator for the second part which is not allowed in the GMAT Exams

If you cannot remember the squares of the first 20 numbers I think the best approach is to play with the units digits only. The root of 324 should end to 2 or 8 and the root of 289 should end to 3 or 7. The next step is to take those figures in pairs and check the digit of their sum. So (2,3) --> units digit 5 (Answer D) (2,7) --> units digit 9 (No Answer) (8,3) --> units digit 1 (No Answer) (8,7) --> units digit 5 (Answer D) I don't even care about which numbers they actually are!

And it works for any number regardless if it is perfect square or not.

Thanks LU

What is this "Math formula" ? It works for any number? The square root of 64 + the square root of 25 is NOT equal to 64 - 25. Or what am I missing here?