一家人过桥问题 有一家人，晚上要过一个独木桥。但是他们只有一盏灯,而这盏灯只能使用30秒。要在灯熄灭前过这座桥不然就过不去了。他们该如何过呢？-There is a problem of a family of people cross the bridge at night to have a single-plank bridge. But they have only a lamp, which lamp can only use 30 seconds. Before the ligh