For 100% efficiency we--------- sent (1+2a) Frames
For 60% efficiency we----------- sent ? Frames
= 60/100(1+2a) , where a = Propogation delay(Tp)/Transmission delay(Tt)
= 11.85
We are using the Selective repeat sliding window protocol. Therefore, Sender window size(Ws) = Reciever window size(Wr).
We know, Available sequences number >= Ws+Wr.
Available sequences number = 11.85 +11.85 = 23.70
Therefore, sequence number bits required = ceil(Log 23.70) = 4 bits.