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 who has failed the test came from the South District?

a) 0.15

b) 0.40

c) 0.50

d) 0.75