$11-24$ Using PERT, Ed Rose was able to determine that the expected project completion time for the construction of a pleasure yacht is $21$ months and the project variance is $4.$

a$.$ What is the probability that the project will be completed in $17$ months or less?

b$.$ What is the probability that the project will be completed in $20$ months or less?

c$.$ What is the probability that the project will be completed in $23$ months or less?

d$.$ What is the probability that the project will be completed in $25$ months or less?