Assume there are four black pens and four yellow pens in an opaque container. You randomly take one pen from the container each time. Let Y be the number of yellow pens you take from the container before you get the first black pen. Find the probability mass function of Y (P(Y=0), P(Y=1), P(Y=2), P(Y=3), P(Y=4) (Plz show all the steps, i will thumb up the answer, thank you:b