I think its similar to gbn protocol where they have mentioned window size which requires sequence no. ,but direct formula wont work here for gate questions.Here the catch is ACKs are always piggybacked which is given in the second part of the question.Now is Acks are piggybacked ,
Sender transmits one packet in 1 ms and receiver in 1 ms.Useful time = 2 ms
Total time from sending one packet from source to destination and back with piggybacked acks will take = 2 ms (total transmission time)+25 ms (sender to receiver propagation time)+25 ms (receiver to sender propagation time for Piggybacked Ack,which contains data as well as ack) = 52 ms
So,2 packets are sent in 52 ms timeframe from two sides,sender first packet reaches receiver in 26 ms after which receiver starts sending its data with ack of the sender added to it.By normal given book formula it is 52 packets we can pack,here its also same but both sender and receiver are packing 52 together 26 from sender,26 from receiver. Efficiency is also better here than normal case.