几道数学DS请教

jwnew

1.If N is subtracted from 31, the result is 14

是如何列式？

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2 what the number of the roses produced by a certain rosebush during 1981?

(1)     For each rose it produces during 1980,the rosebush produced 1.4 roses during 1981.

(2)     In 1981 the rosebush produced exactly 10 more roses than it produces the previous year.

    

THE ANSWER: C(不明白题意)

    

3. What percent of the cost of a certain surgical procedure was covered by employee R’s insurance plan?

(1)     If 80 percent of the cost had been covered by the insurance plan, R would have had to pay $ 250.

(2)     The cost of the surgical procedure was $1250, and the insurance plan covered all but $312.50 of the cost.

    

THE ANSWER: B（为何不是D）

    

4.Let S be the set of all integers from r to t inclusive. If     S1     is the sum of all even integers in S,     S2 is the sum of all odd integers in S, what is the value of S1-S2?

(1)     r is odd and t - r=5

(2)     r=7

    

THE ANSWER: A(怎解？)        

    

5. Three dice, each with faces numbered from 1 through 6, were tossed onto a game board. If one of the dice turned up 4, what was the sum of the numbers that turned up on all three dice?

(1)     The sum of two of the numbers that turned up was 10.

(2)     The sum of two of the numbers that turned up was 11.

    

THE ANSWER: B(怎解？)        

橙子

以下是引用jwnew在2004-3-15 14:15:00的发言：

1.If N is subtracted from 31, the result is 14

是如何列式？

    31-14=N＝17

2 what the number of the roses produced by a certain rosebush during 1981?(1981年某种玫瑰的产量）

(1)     For each rose it produces during 1980,the rosebush produced 1.4 roses during 1981.（相应于1980年每产一支玫瑰，1981年就生产1.4支）

(2)     In 1981 the rosebush produced exactly 10 more roses than it produces the previous year.（1981年比1980年多产了10支玫瑰）

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THE ANSWER: C(不明白题意)

    设1980年产量是x,1981年是y ,条件１，x/y=1/1.4＋条件2 y-x=10，求出y=35,所以选Ｃ

3. What percent of the cost of a certain surgical procedure was covered by employee R’s insurance plan?

(1)     If 80 percent of the cost had been covered by the insurance plan, R would have had to pay $ 250.

(2)     The cost of the surgical procedure was $1250, and the insurance plan covered all but $312.50 of the cost.

    

THE ANSWER: B（为何不是D）

    条件１是假设了问题的答案的一种可能，并不是充分条件．如果是问cost的总数是多少，条件１就是充分条件

4.Let S be the set of all integers from r to t inclusive. If     S1     is the sum of all even integers in S,     S2 is the sum of all odd integers in S, what is the value of S1-S2?

(1)     r is odd and t - r=5

(2)     r=7

    

THE ANSWER: A(怎解？)        

    有疑问from r to t是否可以理解为 successive numbers,如果不是，是否应该选e，如果是，则选A,

5. Three dice, each with faces numbered from 1 through 6, were tossed onto a game board. If one of the dice turned up 4, what was the sum of the numbers that turned up on all three dice?

(1)     The sum of two of the numbers that turned up was 10.

(2)     The sum of two of the numbers that turned up was 11.

    


THE ANSWER: B(怎解？)        




条件１，10=4+6=5+5，所以，１０可能是已知的这个dice加上另外一个掷为６的dice，而第三个则不能确定，也有可能是另外两个dices的总点数．所以条件１不充分
而条件二，因为和为１１＝5+6，只可能是另外两个未知dice抛出来的总和，所以三个dice的点数总和为15选b

请大家继续讨论！谢谢！

jwnew

thanks! you are really good!