Explanation:
well, we need to see how often 12/3 minutes (for one page) fits into 221/2 minutes (for all pages) :
n = 221/2 / 12/3 = (221×3) / (12×2) = 221 / (4×2) = 221/8
the book had n = 221/8 pages = 27.625 ≈ 27 pages.
as parts of a page do not make sense, we only count full pages.