Throughput of sender = size of frame sent by sender/total time
Total time = t1+t2+t3+t4
Where
t1=transmission time of sender to place the frame of 1000B on the channel
t2=propagation time of the frame to reach the receiver
t3=transmission time of receiver to put the acknowledgement of 100B on the channel
t4=propagation time for the acknowledge to reach the sender
t2=t4=100ms (given)
t1=L/B, L=1000B and B=80kbps
So t1=100ms
And t3=La/B' , La=100B and B'=8kbps
t3=100ms
Total time= 4*100ms =400ms
Throughput= 1000B/400ms = 2500B/s