Answer will be 11 .
As size of window is 7 .You will send packets 1,2,3,4,5,6,7. Now, 5th packet will be lost. Now,as 5th ack is lost it will resend 5,6,7,8,9,10 again. Now ,the 5th packet would be no. 7. (6,7,5,6,7) .Now it will resend 7,8,9,10 again. Now , 5th packet would be no 8 (8,9,10,7,8) .So it will resend 8,9,10 again. 5th packet would be (9,10,8,9,10) 10. It will resend 10 again. Now there will be no more 5th packet.So, total transmission will be 21.Extra =21-10=11.