I am stuck on this question, if you could please explain this for me and the answer I would greatly appreciate it.

A course has two teaching assistants. If a student's assignment goes to Adam then there is a 60% chance that they get a grade higher than a B+, and if the assignment goes to Barbie then there is a 50% chance that the grade will be higher than a B+. Suppose that Barbie grades 75% of all the assignments. a. What is the probability that a student will have a grade higher than a B+ and it was marked by Barbie? b. What is the probability that an assignment does have a grade higher than a B+? A/ c. Given that a grade was higher than a B+, what is the probability that Adam marked it? A/ d. Make up another question similar to part c. that would require the use of Bayes Theorem to solve. A/