题干

某单位N名员工参加“社区低碳你我他”活动.他们的年龄在25岁至50岁之间.按年龄分组:第1组[25,30),第2组[30,35),第3组[35,40),第4组[40,45),第5组[45,50],得到的频率分布直方图如图所示.下表是年龄的频率分布表.

区间

[25,30)

[30,35)

[35,40)

[40,45)

[45,50]

人数

25

a

b

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同类题4

Bobby was sitting out in his back yard in the snow. There were a few holes in his shoes. He thought hard, but he could not come up with an idea for his mother's Christmas gift.
Ever since his father had died three years ago, his mother worked nights at the hospital, but the small money she made could hardly be enough for their needs.
Although the family lacked in money and material things, they got more love in the family. The children ran the family when their mother was absent. All three of Bobby's sisters had already made beautiful gifts for their mother.
It was Christmas Eve already, and he had nothing. Bobby started to walk down to the street. He walked from shop to shop. Everything seemed so beautiful and so far away. It was getting dark. Suddenly he found a shiny dime (一角硬币) on the ground. Never has anyone felt so wealthy as Bobby did at the moment. It was enough to buy a rose with one dime.
He went inside a flower shop. When the owner asked if he could help him, Bobby took out the dime and asked if he could buy one flower. The shop owner looked at Bobby, then said, "You just wait here and I’ll see what I can do for you."
The shop owner returned holding a dozen of red roses in his hand. Bobby's heart sank (下沉) as the owner put them gently into a long white box. "That will be a dime, young man," said the shop owner, reaching out his hand for the dime. Slowly, Bobby moved his hand to give the man his dime.
Then the shopkeeper's wife appeared. "Where are the red roses?"
The shop owner said, "A strange thing happened to me this morning. I thought I heard a voice telling me to leave a dozen of my best roses for a special gift. When I saw that little boy tonight, I knew who that voice was."