Go Back N ARQ Question

I am a computer science under graduation student, and was going through some Go Back N ARQ (Computer Networking) videos on YouTube, and got a doubt in a question, which according to me should have a different answer than what the instructor on the video is arriving at (given that no other comment in the comments section of the video raises the same doubt, I am pretty sure I have had some problem in understanding the protocol). I would be greatful for any help. The question goes as follows:

Given a connection oriented communication between two hosts that are following the Go back N protocol, with sender's window = 3, and assuming that every 5th packet transmitted by the sender is lost (no acknowledgements are lost), what are the total number of transmissions required by the sender host? Assume that the packets to be sent are numbered 1-10.

The sequence of transmissions by the sender that I am getting is (final answer = 16):

1,2,3; 4,5,6; 5,6,7; 8,9,10; 8,9,10; 10

Meanwhile, the instructor in the video gets it like this (final answer = 18):

1,2,3,4,5,6,7,5,6,7,8,9,7,8,9,10,9,10

I would appreciate if someone could point out where I am going wrong in understanding the protocol.
Thanks!

1-2-3 is correctly sent and acknowledged so the sender's window is now over 4-5-6.

4 is received correctly and so it is acknowledged. This makes the window (currently over 4-5-6) slide to 5-6-7. However, since 5 was lost, 5 will not be acknowledged. Therefore, despite 6 being well received, 6 will not be acknowledged as well. Meanwhile the sender sends 7 because his window allows him to (it looks like you think it doesn't trigger the sender to send 7). It is received well but the recipient doesn't acknowledge it because it's still waiting for 5 to arrive. The sender's timer goes off and therefore it sends 5-6-7 again... (etc).

If you haven't seen it already, this animation is really helpful.

StackExchange Computer Science Q#59724, answer score: 5