High cholesterol in the blood can cause fatty deposits called plaques
to form on the walls of the blood vessels.  Suppose a plaque reduces
the effective radius of an artery by 25%.  By what percentage does
this partial blockage affect the speed of the blood through the
artery?

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The volume of liquid transported per unit time thru a conduit is equal 
to the product of the speed of the liquid times the area of the conduit.
Constricting the radius of the conduit by 25% reduces the area of the conduit
by 56% thus the speed of the liquid thru the constricted portion of the 
conduit would be 1.78 times the speed of the liquid thru an unconstricted
portion of the conduit.

This isn't a terribly well-posed question, since the correct answer
will depend to a significant degree on factors that are left
indeterminate. I'm referring to the fact that while altering the
cross-sectional area of the artery does tend to make the flow's speed
through that area increase, the attendant increase in frictional force
of the arterial wall on the blood will tend to mediate that speed
increase. Especially in the case of a moderately viscous fluid such as
blood, this additional friction will result in reduction of the fluid
velocity across most, if not all, of the cross sectional area. So the
correct answer must take into account this friction, the level of
which depends on several factors (such as arterial-wall roughness)
which aren't provided in the question.