通讯员从队头到队尾时：s通+s队=l队,即v通t1+v队t1=l队,
得t1=l队/(v通+v队)=200m/(2m/s+4m/s)=(100/3)s=33.3s
从队尾返回队头时：s通-s队=l队,即v通t2-v队t2=l队,
得t2=l队/(v通-v队)=200m/(4m/s-2m/s)=100s;
所以通信员在此过程中用的时间为t=t1+t2=133.3s