I had a first round phone interview for a quant trader position with D.E. Shaw today, and got stumped on what seems like a pretty simple probability question (see below). Do any of you know how to figure it out? I think it deals with combinations.
There are n socks in a closet, and 3 of them are purple. What is the value of n such that when 2 socks are taken randomly from the drawer, the probability that they are both red is 0.5?