Your example sound like you want to do binning of samples; you can definitely not enter ranges on the calculator. Although it sounds like your questions are a little broad for a single Reddit answer, might I be so bold as to do some self-promotion of "Using the TI-84 Plus", 2nd Edition, which explains statistics on the TI-84 Plus CE (and basically everything else you can do with it) in great detail: http://www.amazon.com/dp/1617293156
Try this: 1. Go to the Solver: [MATH][UP] (to go to the bottom of the menu, and the Solver... option), [ENTER]. 2a. If you have a color calculator with E1: and E2: fields in the Solver menu, enter Y1 (from [VARS]->YVars->1:Function->1:Y1 as E1:, and the Y value you want to find in E2:. 2b. If you have a monochrome calculator a 0= prompt, enter Y1-(the Y value you want to find). 3. Press [ENTER], choose an X that you think is near the Y you want, and press [ENTER] again. The calculator should think for a minute, then return a precise X.
Hope this helps. If my Solver... instructions are a little unclear, there's a good guide in section 2.6 of "Using the TI-84 Plus" (http://www.amazon.com/dp/1617293156/)
Take a look at this series (disclaimer: I made it):
They're based on my Using the TI-84 Plus book.