Need details on how you got the answer please. I cannot use excel only a calculator. I will up vote if work is shown. I need A,B, and C answered. I am a little confused on what im looking at

Three recent college graduates have formed a partnership and have opened an advertising firm. Their first project consists of activities listed in the following table. a- Draw the precedence Diagram and fingd the critical path. The critical path is the longest path through the network. b. What is the probability that project is finished in 21 days or less based on critical path analysis. For each activity find the expected time and variance based on the following formulas. t0=(t0+4tm+tp)/6Var=[(tpt0)/6]2 The critical path is the longest path through the network from start to finish. The critical path is DFG with average duration of 21.5 hours and standard deviation of 1.344 hours. The mean of critical path =12+6+3.5=21.5 days The variance of critical path =36/36+4/36+25/36=65/36=1.806 The standard deviation = sqrt (1.806)=1.344 P(X