As the question is saying after a packet loss sender is sending one dummy packet outside the window for ensuring everything is correct
As Every sixth packet is lost then when every six packet is lost why are you not counting the dummy packet as one of the transmission like
1 2 3 4 5 6 7 8 9 D1 6 7 8 9 10 D2 7 8 9 10 11 D3 8 9 10 11 12 D4 9 10 11 12 13 D5 10 11 12 13 14 D6 11 12 13 14 15 D7 12 13 14 15 16 D8 13 14 15 16 17 D9 14 15 16 17 18 D10 15 16 17 18 19 D11 16 17 18 19 20 D12 17 18 19 20 D13 18 19 20 D14 20
Here Di for i=1,2,3...14 are the Dummy packets.
Don’t you think that even if the dummy packet is outside the window it will still be counted every time when a packet is lost due to which when the 2nd time the packet is lost it should be the packet number 7 like i have explained above.
Hence total transmission should be= 86