5.23. The number of users logged onto a system / and the time 7 until the next user logs off have joint probability given by: P[N = n, X s t] = (1 - p)p"-1(1 - end) for n = 1, 2,... t > 0. (a) Sketch the above joint probability. (b) Find the marginal pmf of N. (c) Find the marginal cdf of X. (d) Find P[N 3/x]