题目

 [1]Surveys find that half of American teenagers are missing an average of up to 1.5 hours of sleep every school night. [2]Surveys of American teenagers find that about half of them do not get enough sleep on school nights. They get an average of sixty to ninety minutes less than experts say they need. [3]Experts say teens are biologically programmed to go to sleep later and wake up later than other age groups. Yet many schools start classes as early as seven in the morning. [4]As a result，many students go to class feeling like sixteen­year­old Danny. He is an active teen—excep已知8人组成的抢险小分队中有3名医务人员，将这8人分为A、B两组，每组4人．（Ⅰ）求A组中恰有一名医务人员的概率；（Ⅱ）求A组中至少有两名医务人员的概率．