几道数学题不懂，帮忙解答，O(∩_∩)O谢谢

麦兜woo

1. Joanna bought only $0.15 stamps and $0.29 stamps.How many $0.15 stamps did she buy?







(1)She bought $4.40 worth of stamps.







(2 She bought an equal number of $0.15 stamps and $0.29 stamps.







A. Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.







      B. Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.







      C. BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.







      D. EACH statement ALONE is sufficient.







      E. Statements (1) and (2) TOGETHER are NOT sufficient

      为什么选D呢？

2. For any positive integer n, the sum of the first n positive integers equals [n(n+1)]/2.  What is the sum of all the even integers between 99 and 301?

A. 10,100        B. 20,200             C. 22,650           D. 40,200            E. 45,150
题目都不清楚啥意思，求解释？

3. A positive integer n is said to be “prime-saturated” if the product of all the different positive prime factors of n is less than the square root of n.  What is the greatest two-digit prime-saturated integer?


A. 99

B. 98

C. 97

D. 96

E. 95

答案选D，为什么？




A. 99

B. 98

C. 97

D. 96

E. 95

答案选D，为什么？

snowinhawaii

可以用改进了的枚举法0.29X+0.15Y=4.4
X,Y必须是整数
根据计算（4.4/0.29），X<=15
而因为减下来的余数必须能被0.15整除，所以X只可能是5,10,15中的一个
计算之后发现只有X=10的时候，Y=10也是整数，所以条件一一个就够了

条件二怎么也不可能单独成立的嘛~想都不用想的

maximeason

第二题拙见  100-300 偶数有101个 首相加末项乘以项数除以二 （100+300）101除以二

麦兜woo

第二题拙见  100-300 偶数有101个 首相加末项乘以项数除以二 （100+300）101除以二

-- by 会员 maximeason (2010/9/26 22:04:20)

还是不明白，能先解释下题目的意思吗？