向大家求助数学题！感谢好心的你！

sunnybyy

本人做题时遇到一些问题搞不懂，快要考试了，特此向大家求助，万分感谢！
1、

shipment	Number of defective chips in the shipment	Total number of chips in the shipment
S1	2	5000
S2	5	12000
S3	6	18000
S4	4	16000

A computer chip manufacture expects the ratio of the number of defective chips to the total number of chips in all future shipments to equal the corresponding ratio for shipments s1,s2,s3,and s4 combined, as shown in the table above. What is the expected number of defective chips in a shipment of 60000 chips?
A.14
B.20
C.22
D.24
E.25
答案是D

2、Pumps A,B and C operate at their respective constant rates. Pumps A and B, operating simultaneously, can fill a certain tank in 6/5 hours; Pumps A and C, operating simultaneously, can fill the tank in 3/2 hours; and Pumps B and C, operating simultaneously,can fill the tank in 2 hours. How many hours does it take pumps A,B and C, operating simultaneously, to fill the tank?
A.1/3
B.1/2
C.2/3
D.5/6
E.1
答案是E

3、If n and y are positive integers and 450y=n^3,which of the following must be an integer?
一、y/(3*2^2*5)   二、y/(3^2*2*5)   三、y/(3*2*5^2)
A.None   
B.一
C. 二
D.三
E.all
答案是B

万分感谢！

sunnybyy

难道是我抄错答案了？求大家认证啊，第一题答案到底是啥？怎么做呀？

momohe

第一题死活算出来只有２０。。。可能是我理解错了

第三个我是这样做的　左右两边同时开３次方都为正整数　左边根号下是５×５×３×３×２×Ｙ　要使开出为正整数　Ｙ必须包含　５×３×２×２这个因式

sunnybyy

第一题怎么做啊？帮帮忙吧

sunnybyy

请问第三题的“y=2^2*5*3*k^3 (K为任意正整数）”是怎么得出来的呀？

sunnybyy

呃。。。不好意思，我刚才少打了个0，应该是60000

stoner28

3  把450分解质因数得：2*5^2*3^2
根据题意，y=2^2*5*3*k^3 (K为任意正整数）
所以么，只有一、y/(3*2^2*5)是一定为整数的，答案B

stoner28

2  设工作总量是T，A,B,C的速度分别是VA,VB,VC,然后根据题目列方程
T/(VA+VB)=6/5;  T/(VA+VC)=3/2;  T/(VB+VC)=2
化简一下可得：VA+VB+VC=T
所以T/(VA+VB+VC)=1

stoner28

1 感觉题目有错。。最后是求6000？