Seven terminals in an on-line system are attached to a communications line to the
central computer. Exactly four of these terminals are ready to transmit a
message. Assume that each terminal is equally likely to be in the ready state (i.e.
have a message to transmit). The central computer polls the terminals in a fixed
sequence without repetition. Let X be the # of terminals polled until the first
ready terminal is located (For example if the second terminal polled is ready,
then X=2, etc…)
a) Find the PMF (Probability Mass Function) of X
b) Find the mean and the Variance of X.