Please refer ->
Check slide 11 - 14 (Notice efficiency can not be greater then 1. :)
Mainly for G-back-N proof -->
Efficiency is defined as the ratio of the time required to transmit a packet, TRANSP, to the average time it takes for a packet to be transmitted, E(T),
Efficiency = TRANSP/ E(T)
From the above description we can calculate E(T) as follows:
E(T) = (1 − p)TRANSP + p(time-out + E(T)).
I just share my pain...