Here's a small TI-86 program for twiddling Linux/UNIX permission bits and seeing both the symbolic and numeric results. The simple ones (750, 640, 755, etc.) are easy enough to do in one's head, but it can start to get a bit messy when setuid, setgid, and sticky get involved.
Just copy this to your calculator with TI Connect to install. Use the F1-F5 keys to enter the user/group/other/special submenus, and then toggle the individual bits therein. Results are updated after each change, with the symbolic modes shown on the left, and numeric (suitable for use with chmod) on the right.