6道 PS题 和 DS题，请大家指教

yoo_hoo

1. A boy walking along a road at 3 kilometers per hour is overtaken by a truck traveling at 40 kilometers per hour. If the truck breaks down 1 kilometer beyond where it passes the boy, how many minutes after the breakdown does the boy reach the truck?

答案是 18.5  ，  请问这题说的是什么意思啊，我越读越糊涂， 方程该怎么列呀？

2. How many three-digit numerals begin with a digit that represents a prime number and end with a digit that represents a prime number?

答案是160，  我知道，开始的 那个质数是4个（2、3、5、7），结尾的也是4个，而中间是10个数（0—9），所以就是4*4*10=160， 我这样理解对吗，如果按照排列组合的话，该怎么列呢？

3. If taxi fare were $1.00 for the first 0.2 mile and $0.20 for each 0.2mile there after ,then the taxi fare for a 3-mile ride was

答案是$4.20，可我觉得不对， 应该是1+0.2*（2.8 / 0.2）= 3.8，  是不是答案错了？

4. A milliner bought a job lot of hats, 1/4 of which were brown. The milliner sold 2/3 of the hats including 4/5 of the brown hats. What fraction of the unsold hats were brown?

答案是3 / 20， 本人一遇到这种题就有点晕，请大家不要见笑，请问这题该怎么做？

5. Are there exactly 3 distinct symbols used to create the code words in language Q?

(1) The set of all code words in language Q is the set of all possible distinct horizonal arrangements of one or more symbols,with no repetion.

(2) There are exactly 15 code words in language Q.

答案是C， A 不行吗？

6. What is the value of x^2_y² ?

(1) x+y = 2x

(2) x+y = 0

答案是D， 可我认为答案错了，应该是B， x^2_y² = (x+y)(x-y)  ，答案A可以求出这道题的答案吗？

谢谢大家，希望大家能诲人不倦，好好的帮帮我这个数学不好的人。谢谢

yoo_hoo

怎么没有人回应啊，希望大家热心的指点，在这先谢谢了

abalone

1. A boy walking along a road at 3 kilometers per hour is overtaken by a truck traveling at 40 kilometers per hour. If the truck breaks down 1 kilometer beyond where it passes the boy, how many minutes after the breakdown does the boy reach the truck?

答案是 18.5  ，  请问这题说的是什么意思啊，我越读越糊涂， 方程该怎么列呀？

一个人走路速度3，一个车速度40，车超过人后在超过处1公里外坏掉了，问这人什么时候能赶上车

60*1/3-60*1/40

2. How many three-digit numerals begin with a digit that represents a prime number and end with a digit that represents a prime number?

答案是160，  我知道，开始的 那个质数是4个（2、3、5、7），结尾的也是4个，而中间是10个数（0—9），所以就是4*4*10=160， 我这样理解对吗，如果按照排列组合的话，该怎么列呢？

就这样就可以

3. If taxi fare were $1.00 for the first 0.2 mile and $0.20 for each 0.2mile there after ,then the taxi fare for a 3-mile ride was

答案是$4.20，可我觉得不对， 应该是1+0.2*（2.8 / 0.2）= 3.8，  是不是答案错了？

我也是3.8

4. A milliner bought a job lot of hats, 1/4 of which were brown. The milliner sold 2/3 of the hats including 4/5 of the brown hats. What fraction of the unsold hats were brown?

答案是3 / 20， 本人一遇到这种题就有点晕，请大家不要见笑，请问这题该怎么做？

[(1/4)*(1-4/5)]/(1-2/3)

5. Are there exactly 3 distinct symbols used to create the code words in language Q?

(1) The set of all code words in language Q is the set of all possible distinct horizonal arrangements of one or more symbols,with no repetion.

(2) There are exactly 15 code words in language Q.

答案是C， A 不行吗？

A为什么能推出只有3个

6. What is the value of x^2_y² ?

(1) x+y = 2x

(2) x+y = 0

答案是D， 可我认为答案错了，应该是B， x^2_y² = (x+y)(x-y)  ，答案A可以求出这道题的答案吗？

1. x+y=2x->x-y=0

lotery

第5题的

综合 C 是不是证明了 有15个distinct symbols ?

abalone

以下是引用lotery在2004-4-18 12:39:00的发言：

第5题的

综合 C 是不是证明了 有15个distinct symbols ?

兄弟怎么推的？

lotery

我想

A) Q集合内的都不一样  > 但不知几个

B) Q有15个 > 不知道distinctive否  

综合C 得 Q有15个distinct symbols   不知道这样思考正确否

yoo_hoo

第六题，x+y=2x , 为什么能推出x-y=0 呢？