answer will be 33...
solution is already given by many i m just writing for proper explaination...
as you can see the name suggest itself GO-BACK N where N is window size.In our question N=4 which means whenever a packet is lost you need to GO BACK 4 and retransmit from there again.
according to question every 6th packet is lost
1 2 3 4 5 6(lost) 7 8 9 when 6th packet is lost ,automatically receiver will discard other incoming packets i.e 7, 8 and 9 ....now why only 7, 8 and 9 because when 6th packet was transmitted window contain 4 packets(ws=4) including 6,7,8,9...therefore we need to GO BACK4 i.e upto 6 and retransmit again.....1 2 3 4 5 6 7 8 9 6 7 8 9... now you have to count after first 6 for the 6th packet ...the 6th packet will be lost is (8) i.e 1 2 3 4 5 6 7 8 9 6 7 8(lost ) 9 10 11.. again follow the same procedure... and you get
1 2 3 4 5 6(lost) 7 8 9 6 7 8(lost) 9 10 11 8 9 10(lost) 11 12 13 10 11 12(lost) 13 14 15 12 13 14(lost) 15 14 15