Letency = transmission time + propagation delay + switch delaly( here not considering Ack delay)
2KB here k is taken as 1k=1000
Transmission Delay= L/B = 2KB/100Mbps = 160 microsect
Propagation Delay = 20 km / 2*108 m/sec = 100 microsec
ANS = 160 + 100 + 40 =300 microsec
By considering Ack Delay ans would be = 160 + 200+40 =400 microsec