A computer disk storage device has ten concentric tracks, numbered 1, 2, . . . , 10 from outermost to innermost, and a single access arm. Let that any particular request for data will take the arm to track

. Assume that the tracks accessed in successive seeks are independent. Let X 5 the number of tracks over which the access arm passes during two successive requests (excluding the track that the arm has just left, so possible X values are ). Compute the pmf of X. [Hint:
. After the conditional probability is written in terms of p1, . . . , p10, by the law of total probability, the desired probability is obtained by summing over i.]