Implementation of Go-Back-N ARQ Flow Control Protocol

In Go-Back-N ARQ, if frames 4, 5, and 6 are received successfully, the receiver may send an ACK ___ to the sender.
In a Go-Back-N ARQ, if the window size is 63, what is the range of sequence numbers?
Both Go-Back-N and Selective Repeat use a?
In GBN sender Window size = 10 and Tp = 49.5ms & Tt = 1ms. What is the Efficiency of the protocol?
A sender has a sliding window of size 15. The first 10 frames are sent. How many frames are in the window now?