题干

一个不透明的袋子中装有三个完全相同的小球,分别标有数字3、4、5.从袋子中随机取出一个小球,用小球上的数字作为十位的数字,然后放回;再取出一个小球,用小球上的数字作为个位上的数字,这样组成一个两位数,试问:按这种方法能组成哪些位数?十位上的数字与个位上的数字之和为9的两位数的概率是多少?用列表法或画树状图法加以说明.

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答案(点此获取答案解析)

解:画树状图如下:

共有9种等可能的结果数,即按这种方法能组成的两位数有33,34,35,43,44,4

同类题2

    It snowed again. My seventh Christmas was round the corner. I dreamed of getting the present from Father Christmas until my elder sister dropped the bomb, “There is no Father Christmas.”
I ran to my grandmother’s house, for she always told the truth. Grandma was at home. I told 1 everything. "No Father Christmas?" she was a little 2. “Don't believe it. Put on your coat, and let's go.”
   “Go? Where, Grandma?” I asked.
   “Kerby's Store.”
    As we walked through its doors, Grandma handed me ten dollars and said, "Take the 3 and buy something for someone who needs it. I'll wait for in the car.”Then she turned and walked out of the Kerby's.
    For a few seconds I just stood there, holding the money, wondering 4 to buy, and who to buy it for.
    I thought of 5 I knew: my family, my friends, my neighbors … I suddenly thought of Bobbie Decker, a 6 with bad and messy hair. He sat right behind me 7 Mrs. Pollack's class.
    Bobbie 8 went to any party during the winter. His mother always wrote a note, telling the teacher that he had a cough, 9 we all knew that he didn't have a cough, and he didn't have a coat. I decided to buy Bobbie a red warm coat 10 he would like.
    That evening, Grandma helped me wrap the coat in Christmas paper and ribbons, and 11 “To Bobbie, From Father Christmas” on it. Grandma told me Father Christmas never let people know that he 12 them. Then she drove me over to Bobbie's house, telling that I was helping Father Christmas 13 the presents.
    Grandma parked the car along the street, and we hid behind the trees near Bobbie's house. Then Grandma said to me, “All right, Father Christmas, get going.”
    I took a deep breath, rushed to his front door, put the present down. 14 the door, rang the doorbell and flew back to the safety of the 15 and Grandma. We waited breathlessly in the dark for the front door to open. Finally it did, and there stood Bobbie.
That night, I realized that Father Christmas was alive and well, and we were on his team.