For a PERT activity with optimistic=3 days, most likely=7 days and pessimistic=10 days, expected time is

PERT expected duration: te = (a + 4m + b) / 6, where a = optimistic, m = most likely, b = pessimistic estimate (beta distribution approximation). Standard deviation sigma = (b - a) / 6; variance sigma^2. PERT is used when activity durations are uncertain; CPM uses single deterministic time estimates. Project duration variance = sum of variances on critical path. te=(a+4m+b)/6=(3+4×7+10)/6=6.83 days.

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