A certain project network consists of three paths. The first path has an expected completion time of 15 weeks and a path variance of 5 weeks, the second path has an expected completion time of 10 weeks and a path variance of 3 weeks, and the third path has an expected completion time of 19 weeks and a path variance of 4 weeks. What is the probability of completing this project within 20 weeks (the joint probability of completing all the paths)?