[讨论]急！！！！要上考场了，请教Prep里的数学

各位牛牛，帮我看几道数学题吧。总是算不出正确答案。

If a certain sample of data hase a mean of 20.0 and a standard deviation of 3.0 which of the following values is more than 2.5 standard deviation from the mean?答案是12

A certain list of 100 data has an average (arithmetic mean)

Of 6 and a standard deviation of d,where d is positive,which of the following pairs of data,when added to the list,must result in a list of 102 data with standard deviation less than d?答案是6和6

A certain car averages 25 miles per gallon of gasoline when driven in the city and 40miles per gallon when driven on the highway.According to these rates,which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?答案是36

If m is a positive odd integer,what is the average (arithmetic mean) of a certain set of m integers?

1 The integers in the set are concecutive multiples of 3

2 The median of the set of integers is 33答案是C

There are 15 slate rocks 20 pumice rocks and 10 granita rocks randomly distributed in a certain field.If 2 rocks are to be chosen at random and without replacement, what is the probability that both rocks will be slate rocks?答案是66分支7

Three grades of milk are 1 percent ,2 percent and 3 percent fat by volume.If x gallons of the 1 percent grade ,y gallons of the 2 percent grade,and z gallons of the 3 percent grade are mixed to give x+y+z gallons of a 1.5percent grade,what is x in terms of y and z?答案是y+3Z

For each of her sales,a saleswaman receives a commission equal to 20 percent of the first 500 of the total amount of the sale,plus 30 percent of the total amount of one of her sales was 800,the saleswoman’s commission was approximately what percent of the total amount of the sale?答案是24%

Amy’s grade was the 90^th pencentile of the 80 grades for her class of the 100 grades from another class,19 were higher than Amy’s,and the rest were lower.If no other grade was what percentile of the grades of the two classes combined?答案是85th

In May Mrs.Lee’earnings were 60 percent of the Lee Family’s total income.In June Mrs.Lee earned 20 percent more than in May.If the rest of the family’s income was the same both months,then, in June Mrs.Lee’s earnings were approximately what percent of the Lee family’s total income?答案是72%

5 pieces of wood have an average (arithmetic mean) length of 124 centimeters and a median length of 140 centimeters.What is the maximum possible length , in centimeters,of the shortest picec of wood?答案是100

Marta bought several pencils,if each pencil was a 23-cent pencil or 21-cent pencil,how many 23-cent pencils did Marta buy?

(1)Marta bought a total of 6 pencils

(2)The total value of the  pencils Marta bought was 130cents

答案是B

Is the positive integer n an odd integer?

(1)n+4is a prime number

(2)n+3is not a prime number

答案是A

If a certain sample of data hase a mean of 20.0 and a standard deviation of 3.0 which of the following values is more than 2.5 standard deviation from the mean?答案是12

这道题的其他选项都有什么呀？

我只记下了答案，郁闷。。。

问了好多人，都不知道怎么做

A certain list of 100 data has an average (arithmetic mean)

Of 6 and a standard deviation of d,where d is positive,which of the following pairs of data,when added to the list,must result in a list of 102 data with standard deviation less than d?答案是6和6

这道题 因为standard deviation是算数据和mean之间差平均是多少 以前d>0 表示有偏差 如果在数据里加上6和6，两个等于mean的数 平均的偏差就一定会变小

A certain car averages 25 miles per gallon of gasoline when driven in the city and 40miles per gallon when driven on the highway.According to these rates,which of the following is closest to the number of miles per gallon that the car averages when it is driven 10 miles in the city and then 50 miles on the highway?答案是36

总共60m 总时间10/25+50/40

所以总路程/总时间 算得36.66666... 约等于36

If m is a positive odd integer,what is the average (arithmetic mean) of a certain set of m integers?

1 The integers in the set are concecutive multiples of 3

2 The median of the set of integers is 33答案是C

因为在1，2都成立的情况下 平均数只能是33

There are 15 slate rocks 20 pumice rocks and 10 granita rocks randomly distributed in a certain field.If 2 rocks are to be chosen at random and without replacement, what is the probability that both rocks will be slate rocks?答案是66分支7

15/45*14/44=7/66

If a certain sample of data hase a mean of 20.0 and a standard deviation of 3.0 which of the following values is more than 2.5 standard deviation from the mean?答案是12

有没有人给我说下怎么做啊？？

有没有人给我说下怎么做啊？？