Question 225375
working with a numerical example makes this more straight forward


say 1000 students in the system


"The north district contains 60% of all students" ___ (60%)1000 = 600


"the south district contains 40% of all students" ___ (40%)1000 = 400


"10% of the north district failed" ___ (10%)600 = 60


"15% of the south district failed" ___ (15%)400 = 60


A: 120 out of 1000 failed, so 880 passed (1000-120)
___ probability of a student from the system passing is 880/1000 or .88


B: 120 failed and 60 of those came from South
___ probability of a failed student from South is 60/120 or .5