It is widely known that many drivers on interstate highways
in the United States do not observe the posted
speed limit. Assume that the actual rates of speed
driven by U.S. motorists are normally distributed with
mean mph and standard deviation 5 mph. Given this
information, answer each of the following independent
questions. (Hint: Use Goal Seek in parts a and b, and
use the Solver add-in with no objective in part c. Solver
is usually used to optimize, but it can also be used to
solve equations with multiple unknowns.)
a. If 40% of all U.S. drivers are observed traveling at
65 mph or more, what is the mean ?
b. If 25% of all U.S. drivers are observed traveling at
50 mph or less, what is the mean ?
c. Suppose now that the mean and standard deviation
of this distribution are both unknown. Furthermore, it
is observed that 40% of all U.S. drivers travel at less
than 55 mph and 10% of all U.S. drivers travel at more
than 70 mph. What must and be?
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