Actually the keyword use here is LAN so we use general formula of efficiency
e=1/1+2a , a=PD/TD PD=propagation delay , TD=transmission Delay
as PD=400 us . TD=1000*8 / 10 * 10^6=800 us
e=1/1+2*(400/800)=.5
Effective Data Rate = e*Data Rate = .5*10=5 Mbps .