Full solutions for full problem please!

Consider what is called the "selective repeat ARQ scheme". Assume that each packet lasts P sec., and that the sender and receiver have a buffer of W packets that is large enough so that W times P > S, where S is the time needed for a packet to be correctly acknowledged by the receiver to the sender [see class notes on ARQ analysis!], as shown in the figure below for a buffer of size W = 4. Also assume that W is so large that is in effect infinite, and that the retransmission timeout T is such that T > S, and that the probability that a packet and its ACK are delivered correctly is 1 - p. Show that the efficiency of this ARQ scheme equals 1 - p