哎，继续数学问题

miguel29

1, When the integer n is divided by 6, the remainder is 3, which of folllowing is not a multiple of 6?
答案是 3n  
碰到这种题，都基本上没有什么头绪，求指点

2，which of following fractions has a decimal equivalent is a terminating decimal?
答案是 39/128

3, Erica has $460 in 5 and 10 dollar bills only. If she has fewer 10 than 5 dollar bills, what is the least possible number of 5 dollar bills she could have?
答案是 32
这种least possible 的题目，看见也好无奈

4，The sum of the first 100 positive integer is 5050. What is the sum of the first 200 positive integers?
答案是 20100
根本不知道怎么入手

求高人指点，不要鄙视我，谢谢啦

chinamerica

1, When the integer n is divided by 6, the remainder is 3, which of folllowing is not a multiple of 6?
答案是 3n  
把n写成 n=6k+3
再看选择项 比如说 3n= 18k+9 = 6(3k+1)+3
这个就不能被6整除

2，which of following fractions has a decimal equivalent is a terminating decimal?
答案是 39/128
因为 terminating decimal 等同于 分母只有2和5这样的因子,
i.e., k/(2^m 5^n)
比如说 1／5， 2／15 。。1／64，。。这样的就是terminating decimal..
而 1/3, 1/7, 5/9......就不是terminating…  
因为128＝2^6所以是terminating decimal

3, Erica has $460 in 5 and 10 dollar bills only. If she has fewer 10 than 5 dollar bills, what is the least possible number of 5 dollar bills she could have?
答案是 32
x张5元bill， y张10元bill … 460=5x+10y
因为10元比5元bill少，就是说 y<x
现在问x最少是多少。
显然 y大了x就会小因为5x和10y的和一定，那么x最小的情况就对应y最大了
x,y都要是整数，所以只能是 y=x-1，y=x-2,…..
460=5x+10(x-1)=15x-10    15x= 470 得不到整数解
460=5x+10(x-2)=15x-20      x=32


4，The sum of the first 100 positive integer is 5050. What is the sum of the first 200 positive integers?

答案是 20100

前100个数和是5050，
后100个数每个比前100个大100， 比如说 101=1+100, 102=2+100,…200=100+100..
也就是说后100是 5050＋100（100）＝15050，
总的前200个数的和就是 5050＋15050＝20100

或者lz可以回头复习一下等差数列求和，直接算哈

miguel29

谢谢啊。辛苦了