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I have seen several textbooks used
in middle-school science that draw pictures of this, but fail to count the
odd cable (the one attached to the bottom pulley) as a supporting cable.
Some books even define the mechanical advantage as "twice the number of
times the cable passes through the lower pulley." Of course that "rule" is
only correct if the cable does not attach to the lower pulley, and from what
I have seen "in the field" it is just as likely to attach to the lower one
as to the upper one.
For some reason, few middle-school textbooks give the
simple rule of just counting the number of cables that run to the lower
pulley.