A PERT network has three activities on critical path with mean time 3, 8 and 6 and standard deviations 1, 2 and 2 respectively. The probability that the project will be completed in 20 days is

Answer: Option C

Standard deviation = square root of (variance) = square root of (1 * 1 + 2 * 2 + 2 * 2) = square root of (9) = 3
D = 20
S = 3 + 8 + 6 = 17
Probability $$=\frac{\left(\text{D}-\text{S}\right)}{\text{standard deviation}}=1$$
Now, from Z table probability = 0.84.