Exercise 1: Creating a 4-Disk RAID 6 Array — Possible Solution
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Command:

mdadm --create /dev/md0 --level=6 --raid-devices=4 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1

Explanation:

Per the chapter's own creation examples, --level selects the RAID
level itself (6, matching the level requested) and --raid-devices
gives the total count of member disks (4, matching the four disks
listed: /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1), followed by the
actual device paths in the order they should be added as members. This
follows the exact same pattern as the chapter's own RAID 1 and RAID 5
examples, substituting level=6 and raid-devices=4 for the four given
disks.

Cross-checking usable capacity against fs1-7:

Per fs1-7's own compare-table, RAID 6 provides "(N-2)/N of raw"
usable capacity, since two disks' worth of capacity across the array
is spent on the two distributed parity blocks RAID 6 requires to
survive two simultaneous disk failures. With N = 4 disks here, usable
capacity works out to (4-2)/4 = 2/4 = 50% of the four disks' combined
raw capacity -- equivalent to two disks' worth of usable space out of
the four physical disks provided, with the other two disks' worth of
raw capacity spent on parity distributed across all four members.

This also satisfies fs1-7's own minimum disk count for RAID 6 --
"it needs at least four disks to be worthwhile at all" -- so four
disks is exactly the minimum viable count for this level, not an
arbitrary number.

WHY THIS WORKS AS AN ANSWER
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This constructs the command by directly mapping the scenario's own
disk count and level onto the chapter's own --level/--raid-devices
pattern, then verifies the resulting usable capacity against fs1-7's
own explicit (N-2)/N formula rather than guessing a number.