Many years ago I remember reading a short story about an advanced civilization at war that rediscovers how to do math mentally. The reason I remember it is the guy tells it to government officials who are delighted because it means they can put people inside bombs or missiles instead of computers, which were thought to be more valuable.
I believe this is one of those stories I read in a science fiction anthology my father brought home on one of his business trips in the sixties or seventies.