Ah... good ol' square roots...
Whenever you take the square root of multiplication or division, you can do the square roots separately. So, we can write:
[tex]\sqrt{\frac{100}{121}} = \frac{\sqrt{100}}{\sqrt{121}}[/tex]
This nifty truth allows us to easily find the answer:
[tex]\frac{\sqrt{100}}{\sqrt{121}} = \frac{10}{11}[/tex]
All done!