T.t=2*P.t (T.t=transmission time,P.t=propagation time)
datasize/bandwidth=2*(l/v)
for standard ethernet bandwidth=10mbps.
datasize/10mbps=2*(l/v) [multiply and dic\vide by 100]............(i)
datasize/10mbps=2*(l/100)/100*v.
datasize/1000mbps=2*(l/100)/v..............(ii)
compare (i) and (ii) .to maintain same frame size length should be l/100.
in the problem length =2000meters.
(l/100)=(2000/100)=20m