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.