Hi, I am student in EECS field. My professor ask me to solve probability problem as a test to join his laboratory.

Actually I have around 10 problems, but I can only ask one problem here.

Here is the problem:

Thanks

An urn contains two white and two black balls. We draw balls from the urn randomly and stop after we find a black ball. What is the expectation of the total number of the drawn balls? o 13 0 aviso 0 a 0 very impor 0