Please provide step by step instructions as im super confused

Problem 17-7 (Static) 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. Use Table B. TIME IN DAYS Immediate Predecessor Activity Optimistic 5 A 6 A B C D E F G H I End Most Likely 6 8 8 12 6 6 3 4 7 Pessimistic 7 11 11 15 9 7 9 5 5 2 4 5 D F B H E, G, I 7 5 8 Click here for the Excel Data File a. What is the probability that the project can be completed in 24 days or less? In 21 days or less? (Round your t, and z values to 2 decimal places and "Standard deviation" to 3 decimal places. Round your final answers to 4 decimal places.) Probability Days 24 days or less 21 days or less b. Suppose it is now the end of the seventh day and that activities A and B have been completed while activity D is 50 percent completed. Time estimates for the completion of activity D are 5, 6, and 7. Activities C and H are ready to begin. Determine the probability of finishing the project by day 24 and the probability of finishing by day 21. (Round your intermediate calculations to 3 decimal places and final answers to 4 decimal places.) Probability Day 24 Day 21 c. The partners have decided that shortening the project by two days would be beneficial, as long as it doesn't cost more than $20,000. They have estimated the daily crashing costs for each activity in thousands, as shown in the following table. Which activities should be crashed? Activity C D E F First Crash $8 $10 $9 $7 $8 $7 $6 Second Crash $10 $11 $10 $9 $9 $8 $8 G H I Activity C Activity G