c/o Leacock 'HUMAN INTEREST PUT INTO MATHEMATICS'

So, for all those years of struggling to understand the question, let alone answer it, you are now telling me: it was all person-centred? Plus! Not only do 'A', 'B', and 'C' have names, there's 'D' too?

    'The student of arithmetic who has successfully striven with money sums and fractions, finds himself confronted by an unbroken expanse of questions known as problems. These are short stories of adventure and industry with the end omitted, and though betraying a strong family resemblance, are not without a certain element of romance. 
    The characters in the plot of a problem are three people called A, B, and C. The form of the question is generally of this sort : "A, B, and C do a certain piece of work. A can do as much work in an hour as in two, or C in four. Find how long they work at it." 
    Or this : "A, B, and C are employed to dig a ditch. A can dig as much in one hour as can dig in two, and can dig twice as fast as C. Find how long, etc., etc." 
    Or after this wise: "A lays a wager that he can walk faster than or C. A can walk half as fast again as B, and C is only an indifferent walker. Find how far, and so forth." . . .

    In the early chapters of the arithmetic, their identity is concealed under the names John, William, and Henry, and they wrangle over the division of marbles. In algebra they are often called X, Y, Z. But these are only their Christian names, and they are really the same people.
    Now, to one who has followed the history of these men through countless pages of problems, watched them in their leisure hours dallying with cord wood, and seen their panting sides heave in the full frenzy of filling a cistern with a leak in it, they become something more than mere symbols. They appear as creatures of flesh and blood, living men with their own passions, ambitions and aspirations like the rest of us.'

LEACOCK, S. (1929). HUMAN INTEREST PUT INTO MATHEMATICS. The Mathematics Teacher, 22(5), 302–304. http://www.jstor.org/stable/27951138