Help- I am looking at some employee information.

Firstly I have a column that shows if the employee has a professional
qualification the second column has whether they are current or have left the
company.

a) How can I count the number of employees with professional qualifications
that are current employees?

b) The third column has their length at the company- can someone tell me how
I can average the time that they have been at the company if they have a
qualification and are current?

It feels like I should be able to do this but for some reason I am
struggling..

eg

A B C
D

Bob Yes Current 3.5
Jenny Current 2.7
Sandy Yes Left 6.3
Thomas Current 5.9
Linda Current 3.3

Any help you can give me would be really appreciated

Lal