问题 解答题

从3名男生和2名女生中任选两人参加演讲比赛,试求:

(1)所选2人都是男生的概率;

(2)所选2人恰有1名女生的概率;

(3)所选2人至少有1名女生的概率.

答案

(1)从3名男生和2名女生中任选两人参加演讲比赛,所有的选法共有

C25
=10种,

其中,所选2人都是男生的选法有

C23
=3种,故所选2人都是男生的概率为
3
10

(2)所选2人恰有1名女生的选法有3×2=6 种,所有的选法共有

C25
=10种,由此可得所选2人人恰有1名女生的概率为
6
10
=
3
5

(3)所选2人至少有1名女生的选法有3×2+1=7种,所有的选法共有

C25
=10种,所选2人少有1名女生的概率为
7
10

单项选择题

I was a single parent of four small children, working at a low paid job. Money was always tight, but we had a (1) over our heads, food on the table, clothes on our backs, and if not a lot, always (2) . Not knowing we were poor, my kids just thought I was (3) . I’ve always been glad about that.

It was Christmas time, and although there wasn’t (4) for a lot of gifts, we planned to celebrate with a family party. But the big (5) for the kids was the fun of Christmas (6) .

They planned weeks ahead of time, asking (7) what they wanted for Christmas. Fortunately, I had saved MYM120 for (8) to share by all five of us.

The big (9) arrived. I gave each kid a twenty-dollar bill and (10) them to look for gifts of about four dollars each. Then everyone scattered. We had two hours to shop; then we would (11) back at the "Santa’s Workshop".

Driving home, everyone was in high Christmas spirits, (12) my younger daughter, Ginger, who was unusually (13) . She had only one small, flat bag with a few candies—fiftycent candies! I was so angry, but I didn’t say anything (14) we got home. I called her into my bedroom and closed the door, (15) to be angry again. This is what she told me.

"I was looking (16) thinking of what to buy, and I (17) to read the little cards on the ’Giving Trees’. One was for a little girl, four years old, and all she (18) for Christmas was a doll. So I took the card off the tree and (19) the doll for her. We have so much and she doesn’t have anything. "

I never felt so (20) as I did that day.

12()

A.including 

B.besides 

C.except 

D.regarding

单项选择题