Together with your team, you applied three-point estimation on a critical path which consists of two activities. The following duration uncertainties are all calculated assuming a ±3sigma confidence interval. The duration uncertainty—defined as pessimistic minus optimistic estimate—of the first activity is 18 days; the second estimate has an uncertainty of 24 days. Applying the PERT formula for paths, what is the duration uncertainty of the entire path?

A) 21 days
B) 30 days
C) 42 days
D) No statement is possible from the information given

I know how to calculate PERT and standard deviation, but I have no clue how to get with the two values 18 and 24 to the correct result...

Can someone please clarify?