题目

某商品按每个5元的利润卖出4个的钱数，与按每个20元的利润卖出3个的钱数一样多，这种商品每个成本是多少元? 答案：解：方法一：根据题意存在下面的关系（5元＋成本）×4＝（20元＋成本）×3，经过倒退可以列式子为： (20×3−5×4)÷(4−3)=40 （元），所以成本为40元方法二：成本不变，每件利润多 20−5=15 （元），3件多 15×3=45 （元），多与少恰好相等，少卖1个少45元，原价利润5元+成本，成本为 45−5=40 （元）。(D)You can not see any object unless light from that object gets into your eyes. Some of the things you see give off light of their own. The sun, the stars, a lighted lamp are examples that can be seen by their own light. Such things are luminous. Most of the things you see are not giving off light of their own. They are simply reflecting light that falls on them from the sun or some other luminous bodies. The moon, for example, does not give off any light of its own. It is non-luminous. You see it because sunlight falls on it and some of it reflects in our direction. So moon light is only second hand sunlight.When you look at a book, it sends to your eyes some of the light which falls on it, and you see the book. If light could be kept out from where you are so that there would be no light for the book to reflect, then you could not see the book even with your eyes wide open.Light travels so fast that the time in which it travels from the book you are reading to your eyes is so short as if there were no time at all. Light reaches us from the moon, which is about 380 000 kilometers away, in only a little more than a second.小题1:You can see the book because_______.A．your eyes are close to it B．it reflects some of the sunlightC．it has light of its ownD．your eyesight can get to it小题2:The underlined word “luminous” means_______. A．visibleB．all colorsC．giving off lightD．sunlight小题3:________ have light of their own.A．The sun and the moonB．The stars and the earth C．The sun and the starsD．The moon and the earth小题4:Which of the following is true?A．All the things you can see give off light.B．Light from the book is much shorter than that from the moon.C．The moment you open your eyes the light from the book travels to your eyes.D．Light travels so fast that there is no time for you to read.小题5:This article is probably from _________ .A．a science magazineB．a medical magazineC．a biographyD．a story book.