Test 6 Section 4 - #6 (page 713)

This one is a backsolve.  We have to look at the answer choices, and figure out which one has 3 integer factors: itself, its square root, and 1.  Since EVERY choice is going to have the factors 1 and itself, we're basically looking to see which ones are perfect squares, and then see which don't have any other factors.  

Start by eliminating 33.  That's not a perfect square.  Then eliminate 64 and 100, because they're even, so they're definitely going to have 2 as a factor.  Then you're down to 121 and 81.

121 is 112, and 81 is 92.  Can you think of any factors of 121 other than 1, 11, and 121?  No?  How about 81?  Any factors other than 1, 9, and 81?  How about 3?  Yep, 3 is also a factor, so we can eliminate 81.  We're left with only 121.  Choice A.