Now count the number of arrangements where the most permeable layer is directly above the least permeable layer. Treat the pair (least, most) as a single unit. There are 5 positions this pair can occupy in a vertical stack of 6: positions 1–2, 2–3, ..., 5–6. For each such position, the pair is fixed in order, and the remaining 4 layers can be arranged in: - Crosslake