Get a rate by month by applying function after groupingBy with Java Stream

I have a Exam table like this:

Exam
student_id | exam_date | status

1            2016-01-01  absence
2            2016-01-28  pass

3            2016-02-03  absence
1            2016-02-08  pass
4            2016-02-22  pass
5            2016-02-27  pass
6            2016-02-28  fail

5            2016-03-01  fail
3            2016-03-24  fail
7            2016-03-30  pass

This should be my 'Map of Integer, Decimal' result after stream:

Month, Rate

[1, 1.00]
[2, 0.75]
[3, 0.33]

Rate = pass / pass + fail

For now I have this following code working, but I have to change Collectors.counting() with the Rate formula:

repository.findAllLastYearExames()
.stream().collect(Collectors.groupingBy(Exam::getMonthExam, Collectors.counting()));