.main{ width:960px; 【任务1】margin:0 auto}

.head{ width:200px;height:100px; background:#F90;margin:0 auto}

.left{ width:220px; height:600px; background:#ccc; float:left;【任务2】}

.right{ width:740px; height:600px;background:#FCC; float:right}

.r_sub_left{ width:540px; height:600px; background:#9C3; float:left}

.r_sub_right{ width:200px; height:600px; background:#9FC; float:right;【任务3】}

.footer{ height:50px; background:#9F9; clear:right;【任务4】}