A PERT chart includes the following information:

Activity	Immediate Predecessor	To	Tm	Tp	Expected Time	Variance
A	None	20	30	50	(20+4(30)+50)/6 = 31.7	(50-20)^2/36 = 25.0
B	None	30	70	120	(30+4(70)+120)/6 = 71.7	(120-30)^2/36 = 225.0
C	B	40	60	75	(40+4(60)+75)/6 = 59.2	(75-40)^2/36 = 34.0
D	A	40	50	70	(40+4(50)+70)/6 = 51.7	(70-40)^2/36 = 25.0
E	C	15	30	50	(15+4(30)+50)/6 = 30.8	(50-15)^2/36 = 34.0
F	D and E	20	40	50	(20+4(40)+50)/6 = 38.3	(50-20)^2/36 = 25.0

a. Calculate the expected completion time of the project.

b.Calculate the project variance and standard deviation of the project.

c.What is the probability that the project will be completed in 190 days or less?

d.What is the probability that the project will be completed in 250 days or less?

I know the answers are a) 200, b) 318.1 and 17.8 c) 28.7% and d) 99.7% but I am unsure what calculations are done to get to these answers.