a$.$ Dan will consider this book to be a blockbuster for him if it sells more than $400$ units. What is the probability that Power and Self$-$ Destruction will be a blockbuster?

Z$=(400-200)/80=2\mathrm{.}5$

b$.$ Dan considers a book a $$dog$$ if it sells less than $50$ percent of his mean forecast. What is the probability this expos$$ is a $$dog$?$

c$.$ What is the probability that demand for this book will be within $20$ percent of the mean forecast?