A project manager needs to estimate the completion time for a project using the Program Evaluation and Review Technique $($PERT$).$ The project has the following activities with optimistic $($O$),$ most likely $($M$),$ and pessimistic $($P$)$ time estimates: Activity A $(2,4,6),$ Activity B $(3,5,8),$ Activity C $(1,3,5),$ and Activity D $(4,6,9).$ Calculate the expected time and variance for each activity, and then determine the total project completion time and variance. Provide a detailed explanation using Excel. $[$ SOLVE THE QUESTION USING EXCEL$]$