Word problems in
mathematics having to do with three individuals, called A, B, and C (occasionally D will be called in as well), who are assigned to do "a piece of work", to dig a hole, or to row or walk a certain
distance over a certain
time, with A generally working, rowing, or walking fastest, B somewhat slower, and C (and/or D) relatively sluggishly. A half-step between "story problems" and true abstraction, the details given don't stop these problems, especially when worked en masse, to be phenomenally boring.
A, B, and C's first cropped up in the early years of the 19th century, and as such remained in the hearts and minds of most algebra students for more than a century. Towards the end of their long tenure as pedagogues, Stephen Leacock wrote an amusing biography of these fascinating individuals.