How do you calculate the effort of a pulley?

The general effort force equation for block and tackle to raise or pull a load can be expressed as

S=F/μn

where
S is the effort force,
F is the load force (often the weight of the object to be moved),
μ is the mechanical efficiency of the system (1 in an ideal, massless, frictionless system of pulleys), and
n is the number of ropes between the sets of pulleys.

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form