Let x be a random variable representing movement (in feet per year) of such sand dunes (measured from the crest of the dune). Let us assume that x has a normal distribution with μ = 11 feet per year and σ = 2.9 feet per year.
Under the influence of prevailing wind patterns, what is the probability of each of the following?

a) an escape dune will move a total distance of more than 90 feet in 9 years
b) an escape dune will move a total distance of less than 80 feet in 9 years
c) an escape dune will move a total distance of between 80 and 90 feet in 9 years

Question

Let x be a random variable representing movement (in feet per year) of such sand dunes (measured from the crest of the dune). Let us assume that x has a normal distribution with μ = 11 feet per year and σ = 2.9 feet per year.
Under the influence of prevailing wind patterns, what is the probability of each of the following? 

a) an escape dune will move a total distance of more than 90 feet in 9 years
b) an escape dune will move a total distance of less than 80 feet in 9 years
c) an escape dune will move a total distance of between 80 and 90 feet in 9 years