As a student drives to school, he encounters a traffic signal. This traffic signal stays green for 35 seconds, yellow for 5 seconds, and red for 60 seconds. Assume that the student goes to school each weekday between 8:00 and 8:30. Let X1 be the number of times he encounters a green light, X2 be the number of times he encounters a yellow light, and X1 be the number of times he encounters a red light. Find the joint distribution of X1, Xa, and X3.