问题 填空题

29个零件中有一个是次品(质量稍重),用天平最少称______次保证称出这件次品.

答案

先从29个零件中拿出1个,将剩余的28个分成14、14两组,称量后,若天平平衡,则拿出的那个是次品;

若不平衡,将轻的那14个再分成7、7两组,第二次称量;

进而将轻的那7个分成1、3、3三组,称量3、3两组,若天平平衡,则1个的那个是次品;

若不平衡,将轻的那3个进行第四次称量,即可找出次品.

这样用天平最少称 4次保证称出这件次品.

故答案为:4.

选择题
阅读理解

More and more people around the world are joining in dangerous sports. Some people climbed the highest mountains; some traveled into unknown parts of the world; some sailed small boats across the largest sea. Now some people begin to look for new excitement.

Bungee jumping (蹦极) and motorcycle racing (摩托车赛) are quite dangerous sports. Bungee jumping only lasts a few minutes or even seconds. You jump from a high place, 200 meters above the ground, and there is a rubber band (橡胶带子) tied to your legs. When you jump down, the rubber band pulls you up. About 2,000,000 people around the world have tried bungee jumping.

Why do people join in these dangerous sports? Some scientists say that it is because modern life has become safe and it is not interesting. In the past, people lived in danger. They had to go out and look for food, and life was like a fight but was interesting.

Many people think that there is little excitement in life. They live and work in safe places, buy food in shops, and there are doctors and hospitals to look after them if they become ill.

小题1:People like to join in dangerous sports because _______.

A.they have a lot of free time

B.they can go to the hospital if they are hurt

C.they need excitement

D.they don’t need to look for food小题2:_______ is not a dangerous sport.

A.Climbing the highest mountains

B.Riding on the road

C.Traveling into unknown parts of the world

D.Sailing small boats across the largest sea小题3:In bungee jumping, you _______.

A.jump up as high as you can

B.jump down with a rubber band tied to your legs

C.jump down without a rubber band

D.jump to the ground小题4:In the past, people lived in danger because _______.

A.the living condition is poor

B.there was no doctor and hospital

C.there were many dangerous animals

D.all of above小题5:Which is not TRUE?

A.Everyone’s life is interesting.

B.Many people live and work in safe places.

C.Many people buy food in shops.

D.People can go to see the doctors when they become ill.