The first one had to do with a math teacher who idly mentions he'd sell his soul to complete a problem. The devil appears and they engage in a battle of wits, the stakes being the math teacher's soul. Some of the specifics here elude me, but I distinctly remember a few things. First, the devil had horns and sunglasses and his shirt had text on it that kept changing. Second, the challenge had something to do with the teacher trying to ask a question the devil couldn't answer (I remember the devil giving specific examples of how previous people tried and failed to do so). And finally, I remember the punchline (which I won't spoil).
Now, this could have been on any number of anthology shows. The Twilight Zone, The Outer Limits, one of those old Hitchcock theater thingys... So I searched for "Twighlight Zone devil [punchline]" (where [punchline] represents the punchline I don't want to spoil).
Bingo, the second result in Google is a detailed recounting of the 8 minute episode (spoilers on that page, don't go until you've watched the episode below), which was apparently titled I of Newton. It starred Sherman Hemsley (of The Jeffersons fame) and Ron Glass (of Firefly fame) and there are apparently a bunch of neat references (Dante, for instance). The page also mentions what one of the devil's t-shirts says: "hell is a city much like Newark." Heh.
Update: Knowing the title, finding the episode on YouTube was easy. Enjoy:
We really need more of these anthology shows, but we seem to be moving in the opposite direction of huge, multi-season story arcs rather than anthologies with 8 minute stories (or short story magazines, for that matter). A shame, really, but I could see the format making a comeback someday.