A metropolitan school system consists of two districts-north and south. The north district contains 60% of all students, and the south district contains 40% of all students. A minimum competency test was given to all students. 10% of the north district students failed, and 15% of the south district students failed. What is the probability that a student from the entire school system, chosen at random, has passed?

a) 0.80

b) 0.85

c) 0.88

d) 0.90