Answer:
σ = 13 miles
Step-by-step explanation:
Let us consider X continuous random variable and λ be the parameter of exponential density function.
where E(x) = [tex]\frac{1}{\lambda}[/tex]
where  E(x) = is expected value=13
we have to find λ=[tex]\frac{1}{E(x)}[/tex]
             λ=[tex]\frac{1}{13}[/tex]
             λ=0.076
standard deviation = V(X) = σ =  [tex]\frac{1}{\lambda}[/tex]
now , σ =  [tex]\frac{1}{0.076}[/tex]
     σ = 13 miles is the distance between the two major crack.
