Answer : 20
following diagram shows the transmissions and retransmissions.
I have assumed that the 4th packet in each window get lost. In GO-BACK ARQ the on each successful transmission of the packets, the window slides ahead. But the packet that get lost becomes the earliest outstanding packet in the window . This whole window is again retransmitted on timeout of this earliest outstanding frame timer .
Here after successful transmission of packet 1, 2,3 ..packet 4 got lost and 5 is transmitted successfully. Since packet 4 was the earliest outstanding packet , the whole window consisting of 4 , 5 , 6, 7, 8 need to be transmitted next. Among this 7th packet suffer packet lost (as per the question ).
Now 7 , 8, 9, 10 need to be send. packet 10 now get lost and hence it is retransmitted.
So, the total transmissions need to send the packets 1 ,2 ... 10 = 20 ( count the lines in the image)