How do I know which values they picked from the variance values to calculate months needed and why? Also, why is it 4M in the equation (O+4M+P)/6? Why is it divided by 6?

Acitivty Optimistic Time (months) Most Likely Time (months) Pessimistic Time (months) Mean Variance 3 12 24 12.5 12.25 1 0.25 0.2 0.2 2 0.5 0.3 0.3 12 1 0.5 0.6 12.25 0.542 0.32 0.33 3.36 0.016 0.003 0.004 3 6 A: Concept development Plan Definition B. Define project scope C. Develop broad schedule D. Detailed cost estimates E. Develop staffing Design & Construction F. Detailed Engineering G. Facility construction H. Mobilization of employees 1. Procurement of equipment Start-up and Turnover J. Pre-start-up inspection K. Recruiting and training L. Solving start-up problems M. Centerlining 12 2 8 0.5 1 3.33 13.33 2.08 4.17 24 4 12 0.45 7.11 0.34 3.36 2 3 0.25 0.25 0 0.5 0.5 1 1 1 1 2 4 0.542 0.542 1 1.33 0.016 0.016 0.11 0.44 0 4