This was a short story that I would have read in a big collection during the mid-to-late 1980s.
I'm pretty sure that the main plot involves two people (a man and a woman?) who find themselves unexpectedly in the far future. I think they were placed (by themselves or someone else) in suspended animation for an unexpectedly long time. For some reason, they don't wake up as planned and "sleep" unchanged for millions of years in a cave. (I know this sounds a lot like Buck Rogers, but I think it's just the same trope. Maybe I have this all wrong, and there's some other mode of one-way time travel or its equivalent.)
Humanity is gone, but there are other thinking creatures who live on the surface. The protagonists encounter some of these creatures, which I think were telepathic and vaguely bird-like or bat-like. The creatures communicate with the protagonists mentally, and I think later the protagonists decide to help them in a war against some other kind of competing creature.
The part that stands out is when the protagonists and the creature first establish contact. The creature is trying to gauge the humans' intelligence, and an argument crops up over "two plus three." The humans define the sum as "five," of course, but the creature explains that this is wrong. If "one plus two equals three" then "two plus three equals four" by the logic that if the first integer plus the second integer equals the third integer (i.e. 1+2=3), then the second integer plus the third integer must equal the fourth integer (i.e. 2+3=4). I think the story explains that the creatures had built an entirely different mathematics based on different first principles.
This weird math is the only thing that really stands out about the story, but it's unusual enough that it might suffice to distinguish this from other stories with similar plots and tropes.