The book in question.
From this textbook the page below was taken (my mother found this while cleaning out the crawlspace). The work was from me. I still maintain there is sufficient interpretational ambiguities in problem number five. We call a pair of skates a “pair”, do we not? And one pair is one thing, is it not?
The answer is yes, yes of course.
Now perhaps it was this nascent rebellious streak, now fully widened out and all-encompassing, that explains the predicament (or perhaps “position” is a better word) your author is in. I’ll leave that for you to decide.
Subjective grading strikes again!