The finance department of a particular company has 15 employees, 6 of whom have MBA degrees. Suppose we select three different employees sequentially at random from this department. Determine the probability of the following events. a) The first employee has an MBA, given that there is a total of one MBA among all three employees. b) There are exactly two employees with an MBA, given that the first employee has an MBA. c) The first employee has an MBA, given that there is at least one MBA among all three employees.