In an urn there are 3 white balls 2 black balls and 2 green balls. We draw with replacement and independently, one ball after the other until we get the green ball. Find the expected number of obtained black balls. Formally, if we denote the color of a ball drawn in k-th round by Xk and introduce τ = inf{n > 1 : Xn = green}, then, the question is to calculate