求好心人士能幫忙解釋一下 及算法

zcooles

116.   
A certain junior class has 1,000 students and a certain senior class has 800 students.  Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior.  If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

(A)  
(B)  
(C)  
(D)  
(E)  
題目沒給選項,只給答案是(A)

121.   

A vending machine is designed to dispense 8 ounces of coffee into a cup.  After a test that recorded the number of ounces of coffee in each of 1,000 cups dispensed by the vending machine, the 12 listed amounts, in ounces, were selected from the data.  If the 1,000 recorded amounts have a mean of 8.1 ounces and a standard deviation of 0.3 ounce, how many of the 12 listed amounts are within 1.5 standard deviations of the mean?

(A) Four
(B) Six
(C) Nine
(D) Ten
(E) Eleven

答(E)

fangzep

60/1000*800=3/40000

左边5个,右边5个,加上8.1自己,11个

danni423

7.51 8.22 7.86 8.36
8.09 7.83 8.30 8.01
7.73 8.25 7.96 8.53
A vending machine is designed to dispense 8 ounces of coffee into a cup. After a test that recorded the number of ounces of coffee in each of 1,000 cups dispensed by the vending machine, the 12 listed amounts, in ounces were selected from the data. If the 1,000 recorded amounts have a mean of 8.1 ounces and a standard deviation of 0.3 ounces, how many of the 12 listed amounts are within 1.5 standard deviation of the mean?
4
6
9
10
11
within 1.5 sd means 0.3*1.5=0.45
mean is 8.1
within 1.5 is between (8.1-0.45) and (8.1+0.45) OR:
7.65 < x < 8.55
only one doesn't fit, so 12 - 1 = 11
Eleven is the answer.

lz题目都漏打了,是有数给你选的

painsper

within 1.5 standard deviation of the mean
意思太恶心